diff --git a/app/javascript/controllers/wiki_toolbar_controller.js b/app/javascript/controllers/wiki_toolbar_controller.js
index 8f92b8a..07dad37 100644
--- a/app/javascript/controllers/wiki_toolbar_controller.js
+++ b/app/javascript/controllers/wiki_toolbar_controller.js
@@ -13,7 +13,8 @@ const isMac = /mac|iphone|ipad/i.test(navigator.userAgentData?.platform || navig
 // textarea's position on connect, so that existing views only need to call
 // wikitoolbar_for. The markup inserted by each button is defined by the text
 // formatter in app/javascript/wiki_formatting/<format>.js, which is loaded on
-// connect.
+// connect. Buttons added by plugins are handled by listening to the
+// wiki-toolbar:button event.
 export default class extends Controller {
   static targets = ['editor', 'preview', 'editTab', 'previewTab', 'toolbar', 'precodeMenu', 'tableMenu']
   static values = {
@@ -52,7 +53,12 @@ export default class extends Controller {
 
   async apply({ params: { button } }) {
     await this.syntaxLoaded
-    this.syntax?.[button]?.(this)
+    const action = this.syntax?.[button]
+    if (action) {
+      action(this)
+    } else {
+      this.dispatch('button', { detail: { button, editor: this } })
+    }
   }
 
   async insertPrecode({ params: { lang } }) {
diff --git a/lib/redmine/wiki_toolbar.rb b/lib/redmine/wiki_toolbar.rb
index fef3321..68a1795 100644
--- a/lib/redmine/wiki_toolbar.rb
+++ b/lib/redmine/wiki_toolbar.rb
@@ -23,6 +23,15 @@ module Redmine
   # The toolbar only describes the layout: which buttons are displayed and in
   # which groups. What each button inserts in the text is defined for each text
   # formatter in app/javascript/wiki_formatting/<format>.js.
+  #
+  # Plugins can add a button by extending #button_groups and #labels (and
+  # #shortcuts) of a text formatter toolbar. The button is handled in JavaScript
+  # by listening to the wiki-toolbar:button event, which gives the name of the
+  # button and the editor:
+  #
+  #   document.addEventListener('wiki-toolbar:button', ({ detail: { button, editor } }) => {
+  #     if (button === 'mark') editor.wrap('%{background:yellow}', '%')
+  #   })
   class WikiToolbar
     LABELS = {
       'strong' => :label_wiki_toolbar_strong,
@@ -85,6 +94,16 @@ module Redmine
       raise NotImplementedError
     end
 
+    # Labels of the buttons
+    def labels
+      LABELS
+    end
+
+    # Keys used with Ctrl (Cmd on macOS) to run the buttons
+    def shortcuts
+      SHORTCUTS
+    end
+
     def render(field_id, preview_url)
       tag.div(class: 'jstBlock', hidden: true,
               data: {
@@ -140,7 +159,7 @@ module Redmine
     end
 
     def render_button(name)
-      title = l(LABELS.fetch(name))
+      title = l(labels.fetch(name))
       data =
         if MENUS.include?(name)
           {action: 'wiki-toolbar#toggleMenu', wiki_toolbar_menu_param: name}
@@ -149,7 +168,7 @@ module Redmine
         else
           {action: 'wiki-toolbar#apply', wiki_toolbar_button_param: name}
         end
-      data[:wiki_toolbar_shortcut] = SHORTCUTS[name] if SHORTCUTS[name]
+      data[:wiki_toolbar_shortcut] = shortcuts[name] if shortcuts[name]
 
       tag.button(tag.span(title), type: 'button', class: "jstb_#{name}", title: title, tabindex: 200, data: data)
     end
diff --git a/test/system/wiki_toolbar_test.rb b/test/system/wiki_toolbar_test.rb
index ce6dfc5..9ef2040 100644
--- a/test/system/wiki_toolbar_test.rb
+++ b/test/system/wiki_toolbar_test.rb
@@ -75,6 +75,29 @@ class WikiToolbarTest < ApplicationSystemTestCase
     end
   end
 
+  def test_button_added_by_a_plugin_should_trigger_an_event
+    log_user('jsmith', 'jsmith')
+    visit '/projects/ecookbook/issues/new'
+
+    within 'div.jstBlock' do
+      assert_selector 'button.jstb_strong'
+    end
+    page.execute_script(<<~JS)
+      document.addEventListener('wiki-toolbar:button', ({ detail: { button, editor } }) => {
+        if (button === 'mark') editor.wrap('[', ']')
+      })
+      const button = document.querySelector('.jstb_strong').cloneNode(true)
+      button.className = 'jstb_mark'
+      button.dataset.wikiToolbarButtonParam = 'mark'
+      document.querySelector('.jstElements').append(button)
+    JS
+    fill_in 'Description', :with => 'text'
+    select_text('issue_description', 0, 4)
+    find('button.jstb_mark').click
+
+    assert_equal '[text]', find('#issue_description').value
+  end
+
   def test_insert_table_using_common_mark
     with_settings :text_formatting => 'common_mark' do
       log_user('jsmith', 'jsmith')
diff --git a/test/unit/lib/redmine/wiki_toolbar_test.rb b/test/unit/lib/redmine/wiki_toolbar_test.rb
index 7446a05..cc825e1 100644
--- a/test/unit/lib/redmine/wiki_toolbar_test.rb
+++ b/test/unit/lib/redmine/wiki_toolbar_test.rb
@@ -64,6 +64,27 @@ class Redmine::WikiToolbarTest < Redmine::HelperTest
     end
   end
 
+  def test_button_added_by_a_plugin
+    toolbar_class = Class.new(Redmine::WikiFormatting::Textile::WikiToolbar) do
+      def button_groups
+        super + [%w[mark]]
+      end
+
+      def labels
+        super.merge('mark' => :button_add)
+      end
+
+      def shortcuts
+        super.merge('mark' => 'm')
+      end
+    end
+
+    html = toolbar_class.new(self).render('issue_description', '/preview')
+
+    assert_select_in html, 'button.jstb_mark[title=Add][data-action=?][data-wiki-toolbar-button-param=mark][data-wiki-toolbar-shortcut=m]',
+                     'wiki-toolbar#apply'
+  end
+
   def test_textile_buttons
     with_settings :text_formatting => 'textile' do
       html = wikitoolbar_for('issue_description')