diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index 8960f5ab9..b3a062474 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -672,16 +672,35 @@ module ApplicationHelper
     if collection.include?(User.current)
       s << content_tag('option', "<< #{l(:label_me)} >>", :value => User.current.id)
     end
-    groups = +''
+
+    involved_users_html = +''
+    # This optgroup is displayed only when editing a single issue
+    if @issue.present? && !@issue.new_record?
+      users = [@issue.author, @issue.prior_assigned_to].uniq.compact
+      involved_users_html = users.map do |user|
+        content_tag('option', user.name, value: user.id, disabled: !collection.include?(user))
+      end.join
+    end
+
+    users_html = +''
+    groups_html = +''
     collection.sort.each do |element|
       if option_value_selected?(element, selected) || element.id.to_s == selected
         selected_attribute = ' selected="selected"'
       end
-      (element.is_a?(Group) ? groups : s) <<
+      (element.is_a?(Group) ? groups_html : users_html) <<
         %(<option value="#{element.id}"#{selected_attribute}>#{h element.name}</option>)
     end
-    unless groups.empty?
-      s << %(<optgroup label="#{h(l(:label_group_plural))}">#{groups}</optgroup>)
+    if involved_users_html.blank? && groups_html.blank?
+      s << users_html
+    else
+      [
+        [l(:label_involved_user_plural), involved_users_html],
+        [l(:label_user_plural), users_html],
+        [l(:label_group_plural), groups_html]
+      ].each do |label, options_html|
+        s << %(<optgroup label="#{h(label)}">#{options_html}</optgroup>) if options_html.present?
+      end
     end
     s.html_safe
   end
diff --git a/app/models/issue.rb b/app/models/issue.rb
index 3debf90ec..48a935558 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -925,6 +925,18 @@ class Issue < ApplicationRecord
     result
   end
 
+  # Returns the assignee immediately prior to the current one from the issue history
+  def prior_assigned_to
+    prior_assigned_to_id =
+      journals.joins(:details)
+              .where(details: {prop_key: 'assigned_to_id'})
+              .where.not(details: {old_value: nil})
+              .order(id: :desc)
+              .pick(:old_value)
+
+    prior_assigned_to_id && Principal.find_by(id: prior_assigned_to_id)
+  end
+
   # Returns the initial status of the issue
   # Returns nil for a new issue
   def status_was
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 1584f5ef2..9c2ae7700 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -1145,6 +1145,7 @@ en:
   label_default_query: Default query
   label_edited: Edited
   label_time_by_author: "%{time} by %{author}"
+  label_involved_user_plural: Involved users
 
   button_login: Login
   button_submit: Submit
diff --git a/config/locales/ja.yml b/config/locales/ja.yml
index 8a6757757..b94017aca 100644
--- a/config/locales/ja.yml
+++ b/config/locales/ja.yml
@@ -1445,3 +1445,4 @@ ja:
   setting_issue_done_ratio_interval: 進捗率の選択肢の間隔
   setting_copy_attachments_on_issue_copy: チケットをコピーするとき添付ファイルもコピー
   field_thousands_delimiter: 3桁区切り表示
+  label_involved_user_plural: 関係者
diff --git a/test/helpers/application_helper_test.rb b/test/helpers/application_helper_test.rb
index 6a7ba4b30..642d817a5 100644
--- a/test/helpers/application_helper_test.rb
+++ b/test/helpers/application_helper_test.rb
@@ -2052,10 +2052,16 @@ class ApplicationHelperTest < Redmine::HelperTest
     User.current = nil
     set_language_if_valid 'en'
     users = [User.find(2), Group.find(11), User.find(4), Group.find(10)]
-    assert_equal(
-      %(<option value="2">John Smith</option><option value="4">Robert Hill</option>) +
-      %(<optgroup label="Groups"><option value="10">A Team</option><option value="11">B Team</option></optgroup>),
-      principals_options_for_select(users))
+    result = principals_options_for_select(users)
+
+    assert_select_in result, 'optgroup[label="Users"]' do
+      assert_select 'option[value="2"]', text: 'John Smith'
+      assert_select 'option[value="4"]', text: 'Robert Hill'
+    end
+    assert_select_in result, 'optgroup[label="Groups"]' do
+      assert_select 'option[value="10"]', text: 'A Team'
+      assert_select 'option[value="11"]', text: 'B Team'
+    end
   end
 
   def test_principals_options_for_select_with_empty_collection
@@ -2070,6 +2076,21 @@ class ApplicationHelperTest < Redmine::HelperTest
                    principals_options_for_select(users)
   end
 
+  def test_principals_options_for_select_should_include_author_and_prior_assignee
+    set_language_if_valid 'en'
+    users = [User.find(2), User.find(3), User.find(1)]
+    @issue = Issue.generate!(author_id: 1, assigned_to_id: 2)
+    @issue.init_journal(users.first, 'update')
+    @issue.assigned_to_id = 3
+    @issue.save
+
+    result = principals_options_for_select(users)
+    assert_select_in result, 'optgroup[label="Author / Prior assignee"]' do
+      assert_select 'option:nth-of-type(1)', text: 'Redmine Admin'  # Author
+      assert_select 'option:nth-of-type(2)', text: 'John Smith'     # Prior assignee
+    end
+  end
+
   def test_stylesheet_link_tag_should_pick_the_default_stylesheet
     assert_match 'href="/assets/styles.css"', stylesheet_link_tag("styles")
   end
diff --git a/test/unit/issue_test.rb b/test/unit/issue_test.rb
index ca2531e58..d9933128a 100644
--- a/test/unit/issue_test.rb
+++ b/test/unit/issue_test.rb
@@ -3257,6 +3257,15 @@ class IssueTest < ActiveSupport::TestCase
     assert_equal was_closed_on, issue.closed_on
   end
 
+  def test_prior_assigned_to
+    issue = Issue.generate!(assigned_to_id: 2)
+    issue.init_journal(User.find(2), 'update')
+    issue.assigned_to_id = 3
+    issue.save
+
+    assert_equal User.find(2), issue.prior_assigned_to
+  end
+
   def test_status_was_should_return_nil_for_new_issue
     issue = Issue.new
     assert_nil issue.status_was