diff --git a/app/assets/javascripts/boards/components/board_sidebar.js.es6 b/app/assets/javascripts/boards/components/board_sidebar.js.es6
index 02459722bbf..75dfcb66bb0 100644
--- a/app/assets/javascripts/boards/components/board_sidebar.js.es6
+++ b/app/assets/javascripts/boards/components/board_sidebar.js.es6
@@ -29,6 +29,12 @@
     watch: {
       detail: {
         handler () {
+          if (this.issue.id !== this.detail.issue.id) {
+            $('.js-issue-board-sidebar', this.$el).each((i, el) => {
+              $(el).data('glDropdown').clearMenu();
+            });
+          }
+
           this.issue = this.detail.issue;
         },
         deep: true
diff --git a/app/assets/javascripts/gl_dropdown.js b/app/assets/javascripts/gl_dropdown.js
index cc1c0877cdf..d2f66cf5249 100644
--- a/app/assets/javascripts/gl_dropdown.js
+++ b/app/assets/javascripts/gl_dropdown.js
@@ -512,12 +512,17 @@
 
     // Append the menu into the dropdown
     GitLabDropdown.prototype.appendMenu = function(html) {
+      return this.clearMenu().append(html);
+    };
+
+    GitLabDropdown.prototype.clearMenu = function() {
       var selector;
       selector = '.dropdown-content';
       if (this.dropdown.find(".dropdown-toggle-page").length) {
         selector = ".dropdown-page-one .dropdown-content";
       }
-      return $(selector, this.dropdown).empty().append(html);
+
+      return $(selector, this.dropdown).empty();
     };
 
     GitLabDropdown.prototype.renderItem = function(data, group, index) {
diff --git a/changelogs/unreleased/issue-sidebar-empty-assignee.yml b/changelogs/unreleased/issue-sidebar-empty-assignee.yml
new file mode 100644
index 00000000000..263af75b9e9
--- /dev/null
+++ b/changelogs/unreleased/issue-sidebar-empty-assignee.yml
@@ -0,0 +1,4 @@
+---
+title: Resets assignee dropdown when sidebar is open
+merge_request:
+author:
diff --git a/spec/features/boards/sidebar_spec.rb b/spec/features/boards/sidebar_spec.rb
index 188d33e8ef4..c28bb0dcdae 100644
--- a/spec/features/boards/sidebar_spec.rb
+++ b/spec/features/boards/sidebar_spec.rb
@@ -141,6 +141,36 @@ describe 'Issue Boards', feature: true, js: true do
         end
       end
     end
+
+    it 'resets assignee dropdown' do
+      page.within(first('.board')) do
+        first('.card').click
+      end
+
+      page.within('.assignee') do
+        click_link 'Edit'
+
+        wait_for_ajax
+
+        page.within('.dropdown-menu-user') do
+          click_link user.name
+
+          wait_for_vue_resource
+        end
+
+        expect(page).to have_content(user.name)
+      end
+
+      page.within(first('.board')) do
+        find('.card:nth-child(2)').click
+      end
+
+      page.within('.assignee') do
+        click_link 'Edit'
+
+        expect(page).not_to have_selector('.is-active')
+      end
+    end
   end
 
   context 'milestone' do