diff --git a/app/controllers/watchers_controller.rb b/app/controllers/watchers_controller.rb
index 3f080e96b6..ba64e33e41 100644
--- a/app/controllers/watchers_controller.rb
+++ b/app/controllers/watchers_controller.rb
@@ -120,6 +120,9 @@ class WatchersController < ApplicationController
     scope = nil
     if params[:q].blank? && @project.present?
       scope = @project.users
+    elsif params[:q].blank? && @projects.present?
+      user_ids = @projects.map { |p| p.users.map(&:id) }.flatten.uniq
+      scope = User.where(:id => user_ids)
     else
       scope = User.all.limit(100)
     end
diff --git a/app/views/watchers/_new.html.erb b/app/views/watchers/_new.html.erb
index 3bd7953ac1..e6cb1ca469 100644
--- a/app/views/watchers/_new.html.erb
+++ b/app/views/watchers/_new.html.erb
@@ -17,7 +17,7 @@
   <%= javascript_tag "observeSearchfield('user_search', 'users_for_watcher', '#{ escape_javascript url_for(: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.pluck(:id) : nil),
                  :project_id => @project) }')" %>
 
   <div id="users_for_watcher">
diff --git a/test/functional/watchers_controller_test.rb b/test/functional/watchers_controller_test.rb
index fb96e99b1e..06dc5f7f10 100644
--- a/test/functional/watchers_controller_test.rb
+++ b/test/functional/watchers_controller_test.rb
@@ -244,6 +244,36 @@ class WatchersControllerTest < Redmine::ControllerTest
     assert_select 'input[name=?][value="9"]', 'watcher[user_ids][]'
   end
 
+  def test_autocomplete_for_user_without_keyword_with_multiple_watchables
+    @request.session[:user_id] = 2
+    User.find(2).roles.each { |role| role.add_permission! :add_issue_watchers }
+
+    get :autocomplete_for_user, :params => {:object_type => 'issue', :object_id => [1, 4]}, :xhr => true
+
+    # Return the users of the project to which object_id belongs.
+    # issue 1: eCookbook, issue 4: OnlineStore
+    assert_response :success
+    assert_select 'input', :count => 3
+    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][]'
+  end
+
+  def test_autocomplete_for_user_with_keyword_and_multiple_watchables
+    @request.session[:user_id] = 2
+    User.find(2).roles.each { |role| role.add_permission! :add_issue_watchers }
+
+    get :autocomplete_for_user, :params => {:object_type => 'issue', :object_id => [1, 4], :q => 'mi'}, :xhr => true
+
+    # Ignore the project and return users matching "mi".
+    assert_response :success
+    assert_select 'input', :count => 4
+    assert_select 'input[name=?][value="1"]', 'watcher[user_ids][]'
+    assert_select 'input[name=?][value="2"]', 'watcher[user_ids][]'
+    assert_select 'input[name=?][value="8"]', 'watcher[user_ids][]'
+    assert_select 'input[name=?][value="9"]', 'watcher[user_ids][]'
+  end
+
   def test_search_non_member_on_create
     @request.session[:user_id] = 2
     project = Project.find_by_name("ecookbook")