diff --git a/app/controllers/watchers_controller.rb b/app/controllers/watchers_controller.rb
index 97afffe012..f781673b35 100644
--- a/app/controllers/watchers_controller.rb
+++ b/app/controllers/watchers_controller.rb
@@ -133,12 +133,15 @@ class WatchersController < ApplicationController
   end
 
   def users_for_new_watcher
-    scope = nil
-    if params[:q].blank? && @project.present?
-      scope = @project.principals.assignable_watchers
-    else
-      scope = Principal.assignable_watchers.limit(100)
-    end
+    scope = if params[:q].blank?
+              if @project.present?
+                @project.principals.assignable_watchers
+              elsif @projects.present? && @projects.size > 1
+                Principal.joins(:members).where(:members => { :project_id => @projects }).assignable_watchers.distinct
+              end
+            else
+              Principal.assignable_watchers.limit(100)
+            end
     users = scope.sorted.like(params[:q]).to_a
     if @watchables && @watchables.size == 1
       watchable_object = @watchables.first
diff --git a/app/views/watchers/_new.html.erb b/app/views/watchers/_new.html.erb
index bc08a3322b..dfff5516c0 100644
--- a/app/views/watchers/_new.html.erb
+++ b/app/views/watchers/_new.html.erb
@@ -31,7 +31,7 @@ title =
                  :controller => 'watchers',
                  :action => 'autocomplete_for_user',
                  :object_type => (watchables.present? ? watchables.first.class.name.underscore : nil),
-                 :object_id => (watchables.present? && watchables.size == 1 ? watchables.first.id : nil),
+                 :object_id => (watchables.present? ? watchables.map(&:id) : nil),
                  :project_id => @project
                )
              )}'
diff --git a/test/functional/watchers_controller_test.rb b/test/functional/watchers_controller_test.rb
index 5e399b7294..3d2224af7a 100644
--- a/test/functional/watchers_controller_test.rb
+++ b/test/functional/watchers_controller_test.rb
@@ -543,4 +543,37 @@ class WatchersControllerTest < Redmine::ControllerTest
       assert_response 404
     end
   end
+
+  def test_ajax_url_should_pass_object_id_when_there_are_multiple_objects
+    @request.session[:user_id] = 2
+    get :new, :params => {
+      :object_id => [7, 9],
+      :object_type => 'issue'
+    }, :xhr => true
+    assert_response :success
+
+    assert_match(
+      %r{/watchers/autocomplete_for_user\?object_id%5B%5D=7&object_id%5B%5D=9&object_type=issue},
+      response.body
+    )
+  end
+
+  def test_autocomplete_for_user_with_multiple_projects
+    # 7 => eCookbook
+    # 9 => Private child of eCookbook
+    @request.session[:user_id] = 2
+    get :autocomplete_for_user, :params => {
+      :object_id => [7, 9],
+      :object_type => 'issue'
+    }, :xhr => true
+    assert_response :success
+
+    # All users from two projects eCookbook (7) and Private child of eCookbook (9)
+    assert_select 'input', :count => 5
+    assert_select 'input[name=?][value="1"]', 'watcher[user_ids][]'
+    assert_select 'input[name=?][value="2"]', 'watcher[user_ids][]'
+    assert_select 'input[name=?][value="3"]', 'watcher[user_ids][]'
+    assert_select 'input[name=?][value="8"]', 'watcher[user_ids][]'
+    assert_select 'input[name=?][value="10"]', 'watcher[user_ids][]'
+  end
 end