diff --git a/app/components/DS/category_select.html.erb b/app/components/DS/category_select.html.erb
index 41bb5d4ee..206edb6d4 100644
--- a/app/components/DS/category_select.html.erb
+++ b/app/components/DS/category_select.html.erb
@@ -15,7 +15,7 @@
                      class: "form-field__label" %>
 
       <button type="button"
-              id="<%= field_id %>"
+              id="<%= trigger_id %>"
               class="form-field__input min-h-7 flex items-center gap-2 pr-8"
               data-category-select-target="button"
               data-action="click->category-select#toggle"
diff --git a/app/components/DS/category_select.rb b/app/components/DS/category_select.rb
index f09efa4fa..cf84f3bf0 100644
--- a/app/components/DS/category_select.rb
+++ b/app/components/DS/category_select.rb
@@ -25,6 +25,10 @@ class DS::CategorySelect < DesignSystemComponent
     form.field_id(:category_id)
   end
 
+  def trigger_id
+    "category_id_trigger"
+  end
+
   def menu_id
     "#{field_id}_menu"
   end
diff --git a/app/components/DS/select.html.erb b/app/components/DS/select.html.erb
index 6999d697b..3bd6600af 100644
--- a/app/components/DS/select.html.erb
+++ b/app/components/DS/select.html.erb
@@ -49,7 +49,7 @@
           role="listbox" tabindex="-1">
       <% items.each do |item| %>
         <% is_selected = item[:value] == selected_value %>
-        <% obj = item[:object] %>
+        <% is_child = child?(item) %>
 
         <%# Options use tabindex="-1" always — keyboard focus is managed
             programmatically so listbox items never appear in the Tab
@@ -67,6 +67,10 @@
             <%= helpers.icon("check") %>
           </span>
 
+          <% if is_child %>
+            <%= helpers.icon("corner-down-right", size: "sm") %>
+          <% end %>
+
           <% case variant %>
           <% when :simple %>
             <%= item[:label] %>
diff --git a/app/components/DS/select.rb b/app/components/DS/select.rb
index b19166bfb..a9a934219 100644
--- a/app/components/DS/select.rb
+++ b/app/components/DS/select.rb
@@ -61,6 +61,14 @@ module DS
       obj&.respond_to?(:logo_url) && obj.logo_url.present? ? Setting.transform_brand_fetch_url(obj.logo_url) : nil
     end
 
+    # Returns true if the item represents a child/subcategory of another item
+    # in the list (i.e. has a parent). Used to visually indent hierarchical
+    # items such as categories with subcategories.
+    def child?(item)
+      obj = item[:object]
+      obj&.respond_to?(:parent_id) && obj.parent_id.present?
+    end
+
     private
 
       def normalize_menu_placement(value)
diff --git a/app/controllers/splits_controller.rb b/app/controllers/splits_controller.rb
index e31b64596..1d0e20145 100644
--- a/app/controllers/splits_controller.rb
+++ b/app/controllers/splits_controller.rb
@@ -3,7 +3,7 @@ class SplitsController < ApplicationController
   before_action :require_split_write_permission!, only: %i[create update destroy]
 
   def new
-    @categories = Current.family.categories.alphabetically
+    @categories = grouped_categories
   end
 
   def create
@@ -35,7 +35,7 @@ class SplitsController < ApplicationController
       return
     end
 
-    @categories = Current.family.categories.alphabetically
+    @categories = grouped_categories
     @children = @entry.child_entries.includes(:entryable)
   end
 
@@ -97,4 +97,8 @@ class SplitsController < ApplicationController
     def split_params
       params.require(:split).permit(splits: [ :name, :amount, :category_id, :excluded ])
     end
+
+    def grouped_categories
+      Current.family.categories.alphabetically_by_hierarchy
+    end
 end
diff --git a/app/controllers/transactions_controller.rb b/app/controllers/transactions_controller.rb
index c21d03a9b..23fdf90bf 100644
--- a/app/controllers/transactions_controller.rb
+++ b/app/controllers/transactions_controller.rb
@@ -583,7 +583,7 @@ class TransactionsController < ApplicationController
         .alphabetically
         .includes(:account_providers, logo_attachment: :blob)
         .to_a
-      @categories = Current.family.categories.alphabetically.to_a
+      @categories = Current.family.categories.alphabetically_by_hierarchy.to_a
       @merchants = Current.family.available_merchants_for(Current.user).alphabetically.to_a
       @tags = Current.family.tags.alphabetically.to_a
     end
diff --git a/app/controllers/transfers_controller.rb b/app/controllers/transfers_controller.rb
index eae79b874..5a7f7e54e 100644
--- a/app/controllers/transfers_controller.rb
+++ b/app/controllers/transfers_controller.rb
@@ -11,7 +11,7 @@ class TransfersController < ApplicationController
   end
 
   def show
-    @categories = Current.family.categories.alphabetically
+    @categories = Current.family.categories.alphabetically_by_hierarchy
     @tags = Current.family.tags.alphabetically
 
     # Whether the current user can hit `mark_as_recurring`: feature flag on,
diff --git a/app/helpers/categories_helper.rb b/app/helpers/categories_helper.rb
index 458dae432..87c208665 100644
--- a/app/helpers/categories_helper.rb
+++ b/app/helpers/categories_helper.rb
@@ -20,6 +20,6 @@ module CategoriesHelper
   end
 
   def family_categories
-    [ Category.uncategorized ].concat(Current.family.categories.alphabetically)
+    [ Category.uncategorized ].concat(Current.family.categories.alphabetically_by_hierarchy)
   end
 end
diff --git a/app/models/category.rb b/app/models/category.rb
index 89163f24a..bad3148a1 100644
--- a/app/models/category.rb
+++ b/app/models/category.rb
@@ -123,11 +123,35 @@ class Category < ApplicationRecord
 
     delegate :name, :color, to: :category
 
+    # NOTE: if `categories` is a filtered/partial collection, any child whose
+    # parent isn't included is silently dropped (it's grouped under its
+    # parent_id, but that parent never appears in `roots`). Every current
+    # call site passes the full family category list, so this is latent
+    # today — pass a filtered scope with care.
     def self.for(categories)
       categories_by_parent_id = categories.to_a.group_by(&:parent_id)
 
-      categories_by_parent_id[nil].to_a.map do |category|
-        new(category, categories_by_parent_id[category.id].to_a)
+      roots = categories_by_parent_id[nil].to_a.sort_by { |category| category.name.downcase }
+
+      roots.map do |category|
+        subcategories = categories_by_parent_id[category.id].to_a.sort_by { |sub| sub.name.downcase }
+        new(category, subcategories)
+      end
+    end
+
+    # Builds [label, id] pairs for plain HTML <select> elements, ordered
+    # parent-then-children with children visually indented. Native <select>
+    # options can't render icons, so we use a unicode arrow prefix (regular
+    # leading spaces collapse in <option> text).
+    #
+    # Pass indent: false when the result is used as a display-label lookup
+    # rather than rendered as actual <select> options (e.g. Rule::Action and
+    # Rule::Condition#value_display), so the cosmetic arrow doesn't leak into
+    # plain-text summaries.
+    def self.select_options(categories, indent: true)
+      self.for(categories).flat_map do |group|
+        [ [ group.category.name, group.category.id ] ] +
+          group.subcategories.map { |sub| [ indent ? "↳ #{sub.name}" : sub.name, sub.id ] }
       end
     end
 
diff --git a/app/models/import/category_mapping.rb b/app/models/import/category_mapping.rb
index 56e7fbdd4..a625aa1d4 100644
--- a/app/models/import/category_mapping.rb
+++ b/app/models/import/category_mapping.rb
@@ -25,7 +25,7 @@ class Import::CategoryMapping < Import::Mapping
   end
 
   def selectable_values
-    family_categories = import.family.categories.alphabetically.map { |category| [ category.name, category.id ] }
+    family_categories = Category::Group.select_options(import.family.categories)
 
     unless key.blank?
       family_categories.unshift [ "Add as new category", CREATE_NEW_KEY ]
diff --git a/app/models/rule/action_executor/set_transaction_category.rb b/app/models/rule/action_executor/set_transaction_category.rb
index 689372f00..97d58c5a1 100644
--- a/app/models/rule/action_executor/set_transaction_category.rb
+++ b/app/models/rule/action_executor/set_transaction_category.rb
@@ -4,7 +4,7 @@ class Rule::ActionExecutor::SetTransactionCategory < Rule::ActionExecutor
   end
 
   def options
-    family.categories.alphabetically.pluck(:name, :id)
+    Category::Group.select_options(family.categories, indent: false)
   end
 
   def execute(transaction_scope, value: nil, ignore_attribute_locks: false, rule_run: nil)
diff --git a/app/models/rule/condition_filter/transaction_category.rb b/app/models/rule/condition_filter/transaction_category.rb
index 71f87bbdf..3e74b9d6d 100644
--- a/app/models/rule/condition_filter/transaction_category.rb
+++ b/app/models/rule/condition_filter/transaction_category.rb
@@ -4,7 +4,7 @@ class Rule::ConditionFilter::TransactionCategory < Rule::ConditionFilter
   end
 
   def options
-    family.categories.alphabetically.pluck(:name, :id)
+    Category::Group.select_options(family.categories, indent: false)
   end
 
   def prepare(scope)
diff --git a/app/views/DS/category_select/_option.html.erb b/app/views/DS/category_select/_option.html.erb
index 763ad6a30..d860c5774 100644
--- a/app/views/DS/category_select/_option.html.erb
+++ b/app/views/DS/category_select/_option.html.erb
@@ -4,6 +4,7 @@
         role="option"
         aria-selected="<%= selected %>"
         data-category-select-target="option"
+        data-value="<%= category.id %>"
         data-category-id="<%= category.id %>"
         data-category-name="<%= category.name %>"
         data-category-display-label="<%= category.name %>"
diff --git a/app/views/splits/_category_select.html.erb b/app/views/splits/_category_select.html.erb
index 02d6ab4bb..f9f81952d 100644
--- a/app/views/splits/_category_select.html.erb
+++ b/app/views/splits/_category_select.html.erb
@@ -62,6 +62,7 @@
       </div>
 
       <% categories.each do |category| %>
+        <% is_child = category.parent_id.present? %>
         <% is_selected = category.id == selected_id %>
         <div class="filterable-item text-sm cursor-pointer flex items-center gap-2 px-3 py-2 rounded-lg hover:bg-container-inset-hover <%= "bg-container-inset" if is_selected %>"
              role="option"
@@ -73,6 +74,9 @@
           <span class="check-icon <%= "hidden" unless is_selected %>">
             <%= icon("check") %>
           </span>
+          <% if is_child %>
+            <%= icon("corner-down-right", size: "sm") %>
+          <% end %>
           <%= render partial: "categories/badge", locals: { category: category } %>
         </div>
       <% end %>
diff --git a/app/views/transactions/_form.html.erb b/app/views/transactions/_form.html.erb
index dd38d09d4..02eb479de 100644
--- a/app/views/transactions/_form.html.erb
+++ b/app/views/transactions/_form.html.erb
@@ -18,7 +18,7 @@
     <% if @entry.account_id %>
       <%= f.hidden_field :account_id, data: { transaction_form_target: "account" } %>
     <% else %>
-      <%= f.collection_select :account_id, manual_accounts, :id, :name, { prompt: t(".account_prompt"), label: t(".account"), selected: Current.user.default_account_for_transactions&.id, variant: :logo }, required: true, class: "form-field__input text-ellipsis", data: { transaction_form_target: "account", action: "change->transaction-form#checkCurrencyDifference" } %>
+      <%= f.collection_select :account_id, manual_accounts, :id, :name, { prompt: t(".account_prompt"), label: t(".account"), selected: Current.user.default_account_for_transactions&.id, variant: :logo, searchable: true }, required: true, class: "form-field__input text-ellipsis", data: { transaction_form_target: "account", action: "change->transaction-form#checkCurrencyDifference" } %>
     <% end %>
 
     <%= f.money_field :amount,
diff --git a/app/views/transactions/bulk_updates/new.html.erb b/app/views/transactions/bulk_updates/new.html.erb
index f899da713..df7d55b37 100644
--- a/app/views/transactions/bulk_updates/new.html.erb
+++ b/app/views/transactions/bulk_updates/new.html.erb
@@ -11,7 +11,7 @@
         <%= render DS::Disclosure.new(title: t(".transactions_section"), open: true) do %>
           <div class="space-y-2">
             <%= form.text_field :name, label: t(".name_label"), placeholder: t(".name_placeholder") %>
-            <%= form.collection_select :category_id, Current.family.categories.alphabetically, :id, :name, { prompt: t(".category_prompt"), label: t(".category_label"), class: "text-subdued", variant: :badge, searchable: true } %>
+            <%= form.collection_select :category_id, Current.family.categories.alphabetically_by_hierarchy, :id, :name, { prompt: t(".category_prompt"), label: t(".category_label"), class: "text-subdued", variant: :badge, searchable: true } %>
             <%= form.collection_select :merchant_id, Current.family.available_merchants_for(Current.user).alphabetically, :id, :name, { prompt: t(".merchant_prompt"), label: t(".merchant_label"), class: "text-subdued" } %>
             <%= form.select :tag_ids, Current.family.tags.alphabetically.pluck(:name, :id), { include_blank: t(".none"), multiple: true, label: t(".tags_label"), include_hidden: false } %>
             <%= form.text_area :notes, label: t(".notes_label"), placeholder: t(".notes_placeholder"), rows: 5 %>
diff --git a/app/views/transactions/categorizes/_entry_row.html.erb b/app/views/transactions/categorizes/_entry_row.html.erb
index 16180bdc3..ff3a0d2a5 100644
--- a/app/views/transactions/categorizes/_entry_row.html.erb
+++ b/app/views/transactions/categorizes/_entry_row.html.erb
@@ -11,7 +11,7 @@
       <%= format_money(entry.amount_money.abs) %>
     </span>
     <%= select_tag "category_id",
-          options_from_collection_for_select(categories, :id, :name),
+          options_for_select(Category::Group.select_options(categories)),
           prompt: t("transactions.categorizes.show.assign_category_prompt"),
           aria: { label: t("transactions.categorizes.entry_row.assign_category_select", name: entry.name) },
           class: "w-full text-xs border border-primary rounded-lg px-1.5 py-0.5 bg-container text-secondary",
diff --git a/app/views/transactions/searches/filters/_category_filter.html.erb b/app/views/transactions/searches/filters/_category_filter.html.erb
index 080bbe3ce..877cde855 100644
--- a/app/views/transactions/searches/filters/_category_filter.html.erb
+++ b/app/views/transactions/searches/filters/_category_filter.html.erb
@@ -11,6 +11,7 @@
       ) %>
   <div class="my-2" id="list" data-list-filter-target="list">
     <% family_categories.each do |category| %>
+      <% is_child = category.parent_id.present? %>
       <div class="filterable-item flex items-center gap-2 p-2" data-filter-name="<%= category.display_name %>">
         <%= form.check_box :categories,
                            {
@@ -20,7 +21,10 @@
                            },
                            category.name,
                            nil %>
-        <%= form.label :categories, category.display_name, value: category.name, class: "text-sm text-primary cursor-pointer" do %>
+        <%= form.label :categories, category.display_name, value: category.name, class: "text-sm text-primary cursor-pointer flex items-center gap-1" do %>
+          <% if is_child %>
+            <%= icon("corner-down-right", size: "sm") %>
+          <% end %>
           <%= render partial: "categories/badge", locals: { category: category } %>
         <% end %>
       </div>
diff --git a/app/views/transactions/show.html.erb b/app/views/transactions/show.html.erb
index 77a00aa5f..132ffff80 100644
--- a/app/views/transactions/show.html.erb
+++ b/app/views/transactions/show.html.erb
@@ -85,7 +85,7 @@
             <%= f.fields_for :entryable do |ef| %>
               <%= render DS::CategorySelect.new(
                     form: ef,
-                    categories: Current.family.categories.alphabetically,
+                    categories: Current.family.categories.alphabetically_by_hierarchy,
                     selected_id: ef.object.category_id,
                     disabled: @entry.split_child? || annotate_locked,
                     auto_submit: true,
diff --git a/app/views/transfers/show.html.erb b/app/views/transfers/show.html.erb
index 42399a8ce..2da9e71ba 100644
--- a/app/views/transfers/show.html.erb
+++ b/app/views/transfers/show.html.erb
@@ -84,7 +84,7 @@
       <%= styled_form_with model: @transfer,
               data: { controller: "auto-submit-form" }, class: "space-y-2" do |f| %>
         <% if @transfer.categorizable? %>
-          <%= f.collection_select :category_id, @categories.alphabetically, :id, :name, { label: t(".category"), include_blank: t(".uncategorized"), selected: @transfer.outflow_transaction.category&.id, variant: :badge, searchable: true }, "data-auto-submit-form-target": "auto" %>
+          <%= f.collection_select :category_id, @categories, :id, :name, { label: t(".category"), include_blank: t(".uncategorized"), selected: @transfer.outflow_transaction.category&.id, variant: :badge, searchable: true }, "data-auto-submit-form-target": "auto" %>
         <% end %>
         <%= render DS::TagSelect.new(
               form: f,
diff --git a/test/components/DS/select_test.rb b/test/components/DS/select_test.rb
new file mode 100644
index 000000000..f3011bc71
--- /dev/null
+++ b/test/components/DS/select_test.rb
@@ -0,0 +1,49 @@
+require "test_helper"
+
+class DS::SelectTest < ViewComponent::TestCase
+  test "child? is true for an item whose object has a present parent_id" do
+    select = DS::Select.new(
+      form: nil,
+      method: :category_id,
+      items: [ categories(:subcategory) ]
+    )
+
+    item = select.items.first
+    assert select.child?(item)
+  end
+
+  test "child? is false for an item whose object has no parent_id" do
+    select = DS::Select.new(
+      form: nil,
+      method: :category_id,
+      items: [ categories(:food_and_drink) ]
+    )
+
+    item = select.items.first
+    assert_not select.child?(item)
+  end
+
+  test "child? is false for objects that don't respond to parent_id" do
+    select = DS::Select.new(
+      form: nil,
+      method: :merchant_id,
+      items: [ merchants(:netflix) ]
+    )
+
+    item = select.items.first
+    assert_not select.child?(item)
+  end
+
+  test "child? is false for the include_blank placeholder item" do
+    select = DS::Select.new(
+      form: nil,
+      method: :category_id,
+      items: [ categories(:subcategory) ],
+      include_blank: "Uncategorized"
+    )
+
+    blank_item = select.items.first
+    assert_nil blank_item[:object]
+    assert_not select.child?(blank_item)
+  end
+end
diff --git a/test/controllers/splits_controller_test.rb b/test/controllers/splits_controller_test.rb
index 5cb89a9ff..412ef7aef 100644
--- a/test/controllers/splits_controller_test.rb
+++ b/test/controllers/splits_controller_test.rb
@@ -17,6 +17,21 @@ class SplitsControllerTest < ActionDispatch::IntegrationTest
     assert_response :success
   end
 
+  test "new groups subcategories immediately after their parent in the category selector" do
+    get new_transaction_split_path(@entry)
+    assert_response :success
+
+    doc = Nokogiri::HTML::Document.parse(response.body)
+    category_values = doc.css(".category-select-container [data-value]").map { |node| node["data-value"] }
+
+    parent_index = category_values.index(categories(:food_and_drink).id)
+    child_index = category_values.index(categories(:subcategory).id)
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
   test "create with valid params splits transaction" do
     assert_difference "Entry.count", 2 do
       post transaction_split_path(@entry), params: {
diff --git a/test/controllers/transactions/categorizes_controller_test.rb b/test/controllers/transactions/categorizes_controller_test.rb
index 1e74d8bbb..8c40f3286 100644
--- a/test/controllers/transactions/categorizes_controller_test.rb
+++ b/test/controllers/transactions/categorizes_controller_test.rb
@@ -26,6 +26,23 @@ class Transactions::CategorizesControllerTest < ActionDispatch::IntegrationTest
     assert_response :success
   end
 
+  test "show groups subcategories immediately after their parent in the category select" do
+    create_transaction(account: @account, name: "Starbucks")
+    get transactions_categorize_url
+
+    assert_response :success
+
+    doc = Nokogiri::HTML::Document.parse(response.body)
+    option_values = doc.css("select[name='category_id'] option").map { |node| node["value"] }
+
+    parent_index = option_values.index(categories(:food_and_drink).id)
+    child_index = option_values.index(categories(:subcategory).id)
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
   test "show renders full dates so multi-year lists are unambiguous" do
     create_transaction(account: @account, name: "Starbucks", date: Date.new(2024, 7, 8))
 
diff --git a/test/controllers/transactions_controller_test.rb b/test/controllers/transactions_controller_test.rb
index 045a54b10..2ee4d1414 100644
--- a/test/controllers/transactions_controller_test.rb
+++ b/test/controllers/transactions_controller_test.rb
@@ -8,6 +8,21 @@ class TransactionsControllerTest < ActionDispatch::IntegrationTest
     @entry = entries(:transaction)
   end
 
+  test "index groups subcategories immediately after their parent in the category filter" do
+    get transactions_url
+    assert_response :success
+
+    doc = Nokogiri::HTML::Document.parse(response.body)
+    checkbox_values = doc.css("input[name='q[categories][]']").map { |node| node["value"] }
+
+    parent_index = checkbox_values.index(categories(:food_and_drink).name)
+    child_index = checkbox_values.index(categories(:subcategory).name)
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
   test "creates with transaction details" do
     assert_difference [ "Entry.count", "Transaction.count" ], 1 do
       post transactions_url, params: {
@@ -698,6 +713,37 @@ end
     assert_not entry.protected_from_sync?
   end
 
+  test "new groups subcategories immediately after their parent in the category select" do
+    get new_transaction_url
+    assert_response :success
+
+    doc = Nokogiri::HTML::Document.parse(response.body)
+    trigger = doc.at_css("#category_id_trigger")
+    assert_not_nil trigger, "expected the category select trigger button to render"
+
+    wrapper = trigger.ancestors(".relative").first
+    category_values = wrapper.css("[data-value]").map { |node| node["data-value"] }
+
+    parent_index = category_values.index(categories(:food_and_drink).id)
+    child_index = category_values.index(categories(:subcategory).id)
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
+  test "new renders a search box for account selection" do
+    get new_transaction_url
+    assert_response :success
+
+    doc = Nokogiri::HTML::Document.parse(response.body)
+    trigger = doc.at_css("#account_id_trigger")
+    assert_not_nil trigger, "expected the account select trigger button to render"
+
+    wrapper = trigger.ancestors(".relative").first
+    assert_not_nil wrapper.at_css("input[type='search']"), "expected a search input inside the account select"
+  end
+
   test "new with duplicate_entry_id pre-fills form from source transaction" do
     @entry.reload
 
diff --git a/test/models/category/group_test.rb b/test/models/category/group_test.rb
new file mode 100644
index 000000000..d41de5726
--- /dev/null
+++ b/test/models/category/group_test.rb
@@ -0,0 +1,58 @@
+require "test_helper"
+
+class Category::GroupTest < ActiveSupport::TestCase
+  setup do
+    @family = families(:dylan_family)
+  end
+
+  test "for groups categories by parent, root categories without children get an empty subcategories array" do
+    groups = Category::Group.for(@family.categories)
+
+    food_group = groups.find { |g| g.category == categories(:food_and_drink) }
+    assert_equal [ categories(:subcategory) ], food_group.subcategories
+
+    income_group = groups.find { |g| g.category == categories(:income) }
+    assert_equal [], income_group.subcategories
+  end
+
+  test "for does not include a category as its own root when it is a subcategory" do
+    groups = Category::Group.for(@family.categories)
+
+    assert_nil groups.find { |g| g.category == categories(:subcategory) }
+  end
+
+  test "select_options orders parent immediately followed by its children" do
+    options = Category::Group.select_options(@family.categories)
+
+    parent_index = options.index { |label, id| id == categories(:food_and_drink).id }
+    child_index = options.index { |label, id| id == categories(:subcategory).id }
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index, "subcategory should immediately follow its parent in the options list"
+  end
+
+  test "select_options indents subcategory labels so they're visually distinguishable from parents" do
+    options = Category::Group.select_options(@family.categories)
+
+    parent_label, = options.find { |label, id| id == categories(:food_and_drink).id }
+    child_label, = options.find { |label, id| id == categories(:subcategory).id }
+
+    assert_equal categories(:food_and_drink).name, parent_label
+    assert_equal "↳ #{categories(:subcategory).name}", child_label
+  end
+
+  test "select_options orders roots and subcategories alphabetically regardless of input order" do
+    reversed_options = Category::Group.select_options(@family.categories.to_a.reverse)
+    forward_options = Category::Group.select_options(@family.categories)
+
+    assert_equal forward_options, reversed_options
+  end
+
+  test "select_options returns [label, id] pairs usable directly by options_for_select / form.select" do
+    options = Category::Group.select_options(@family.categories)
+
+    assert options.all? { |pair| pair.is_a?(Array) && pair.size == 2 }
+    assert options.all? { |_label, id| id.present? }
+  end
+end
diff --git a/test/models/category_test.rb b/test/models/category_test.rb
index 637546a56..c227b1ebc 100644
--- a/test/models/category_test.rb
+++ b/test/models/category_test.rb
@@ -160,6 +160,17 @@ class CategoryTest < ActiveSupport::TestCase
     assert_not lookup.key?(0)
   end
 
+  test "alphabetically_by_hierarchy orders a category's subcategories immediately after it" do
+    ordered = @family.categories.alphabetically_by_hierarchy.to_a
+
+    parent_index = ordered.index(categories(:food_and_drink))
+    child_index = ordered.index(categories(:subcategory))
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
   test "recently_used_for orders by last_used_at, most recent first" do
     older = categories(:income)
     newer = categories(:food_and_drink)
diff --git a/test/models/import/category_mapping_test.rb b/test/models/import/category_mapping_test.rb
new file mode 100644
index 000000000..4d20966b1
--- /dev/null
+++ b/test/models/import/category_mapping_test.rb
@@ -0,0 +1,25 @@
+require "test_helper"
+
+class Import::CategoryMappingTest < ActiveSupport::TestCase
+  setup do
+    @mapping = import_mappings(:one)
+  end
+
+  test "selectable_values groups subcategories immediately after their parent" do
+    options = @mapping.selectable_values
+
+    parent_index = options.index { |_label, id| id == categories(:food_and_drink).id }
+    child_index = options.index { |_label, id| id == categories(:subcategory).id }
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
+  test "selectable_values still prepends 'Add as new category' when key is present" do
+    options = @mapping.selectable_values
+
+    assert_equal "Add as new category", options.first.first
+    assert_equal Import::Mapping::CREATE_NEW_KEY, options.first.last
+  end
+end
diff --git a/test/models/rule/action_executor/set_transaction_category_test.rb b/test/models/rule/action_executor/set_transaction_category_test.rb
new file mode 100644
index 000000000..415c52fe6
--- /dev/null
+++ b/test/models/rule/action_executor/set_transaction_category_test.rb
@@ -0,0 +1,29 @@
+require "test_helper"
+
+class Rule::ActionExecutor::SetTransactionCategoryTest < ActiveSupport::TestCase
+  setup do
+    @rule = rules(:one)
+    @executor = Rule::ActionExecutor::SetTransactionCategory.new(@rule)
+  end
+
+  test "options groups subcategories immediately after their parent" do
+    options = @executor.options
+
+    parent_index = options.index { |_label, id| id == categories(:food_and_drink).id }
+    child_index = options.index { |_label, id| id == categories(:subcategory).id }
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
+  test "options does not indent subcategory labels, so value_display doesn't show the arrow" do
+    action = Rule::Action.new(
+      rule: @rule,
+      action_type: "set_transaction_category",
+      value: categories(:subcategory).id
+    )
+
+    assert_equal categories(:subcategory).name, action.value_display
+  end
+end
diff --git a/test/models/rule/condition_filter/transaction_category_test.rb b/test/models/rule/condition_filter/transaction_category_test.rb
new file mode 100644
index 000000000..b458dad3b
--- /dev/null
+++ b/test/models/rule/condition_filter/transaction_category_test.rb
@@ -0,0 +1,27 @@
+require "test_helper"
+
+class Rule::ConditionFilter::TransactionCategoryTest < ActiveSupport::TestCase
+  setup do
+    @rule = rules(:one)
+    @filter = Rule::ConditionFilter::TransactionCategory.new(@rule)
+  end
+
+  test "options groups subcategories immediately after their parent" do
+    options = @filter.options
+
+    parent_index = options.index { |_label, id| id == categories(:food_and_drink).id }
+    child_index = options.index { |_label, id| id == categories(:subcategory).id }
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
+  test "options does not indent subcategory labels, since this array doubles as the value_display lookup" do
+    options = @filter.options
+
+    child_label, = options.find { |_label, id| id == categories(:subcategory).id }
+
+    assert_equal categories(:subcategory).name, child_label
+  end
+end