diff --git a/CHANGELOG b/CHANGELOG
index 00b97d3c14f..fad784cc385 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -34,6 +34,7 @@ v 8.11.0 (unreleased)
   - Fix awardable button mutuality loading spinners (ClemMakesApps)
   - Add support for using RequestStore within Sidekiq tasks via SIDEKIQ_REQUEST_STORE env variable
   - Optimize maximum user access level lookup in loading of notes
+  - Send notification emails to users newly mentioned in issue and MR edits !5800
   - Add "No one can push" as an option for protected branches. !5081
   - Improve performance of AutolinkFilter#text_parse by using XPath
   - Add experimental Redis Sentinel support !1877
diff --git a/app/mailers/emails/issues.rb b/app/mailers/emails/issues.rb
index 6f54c42146c..d64e48f774b 100644
--- a/app/mailers/emails/issues.rb
+++ b/app/mailers/emails/issues.rb
@@ -6,6 +6,11 @@ module Emails
       mail_new_thread(@issue, issue_thread_options(@issue.author_id, recipient_id))
     end
 
+    def new_mention_in_issue_email(recipient_id, issue_id, updated_by_user_id)
+      setup_issue_mail(issue_id, recipient_id)
+      mail_answer_thread(@issue, issue_thread_options(updated_by_user_id, recipient_id))
+    end
+
     def reassigned_issue_email(recipient_id, issue_id, previous_assignee_id, updated_by_user_id)
       setup_issue_mail(issue_id, recipient_id)
 
diff --git a/app/mailers/emails/merge_requests.rb b/app/mailers/emails/merge_requests.rb
index 9dd11d20ea6..95810b0ac6e 100644
--- a/app/mailers/emails/merge_requests.rb
+++ b/app/mailers/emails/merge_requests.rb
@@ -6,6 +6,11 @@ module Emails
       mail_new_thread(@merge_request, merge_request_thread_options(@merge_request.author_id, recipient_id))
     end
 
+    def new_mention_in_merge_request_email(recipient_id, merge_request_id, updated_by_user_id)
+      setup_merge_request_mail(merge_request_id, recipient_id)
+      mail_answer_thread(@merge_request, merge_request_thread_options(updated_by_user_id, recipient_id))
+    end
+
     def reassigned_merge_request_email(recipient_id, merge_request_id, previous_assignee_id, updated_by_user_id)
       setup_merge_request_mail(merge_request_id, recipient_id)
 
diff --git a/app/services/issuable_base_service.rb b/app/services/issuable_base_service.rb
index 2d96efe1042..b0ea7c905f8 100644
--- a/app/services/issuable_base_service.rb
+++ b/app/services/issuable_base_service.rb
@@ -104,11 +104,12 @@ class IssuableBaseService < BaseService
     change_subscription(issuable)
     filter_params
     old_labels = issuable.labels.to_a
+    old_mentioned_users = issuable.mentioned_users.to_a
 
     if params.present? && update_issuable(issuable, params)
       issuable.reset_events_cache
       handle_common_system_notes(issuable, old_labels: old_labels)
-      handle_changes(issuable, old_labels: old_labels)
+      handle_changes(issuable, old_labels: old_labels, old_mentioned_users: old_mentioned_users)
       issuable.create_new_cross_references!(current_user)
       execute_hooks(issuable, 'update')
     end
diff --git a/app/services/issues/update_service.rb b/app/services/issues/update_service.rb
index c7d406cc331..a2111b3806b 100644
--- a/app/services/issues/update_service.rb
+++ b/app/services/issues/update_service.rb
@@ -4,7 +4,7 @@ module Issues
       update(issue)
     end
 
-    def handle_changes(issue, old_labels: [])
+    def handle_changes(issue, old_labels: [], old_mentioned_users: [])
       if has_changes?(issue, old_labels: old_labels)
         todo_service.mark_pending_todos_as_done(issue, current_user)
       end
@@ -32,6 +32,11 @@ module Issues
       if added_labels.present?
         notification_service.relabeled_issue(issue, added_labels, current_user)
       end
+
+      added_mentions = issue.mentioned_users - old_mentioned_users
+      if added_mentions.present?
+        notification_service.new_mentions_in_issue(issue, added_mentions, current_user)
+      end
     end
 
     def reopen_service
diff --git a/app/services/merge_requests/update_service.rb b/app/services/merge_requests/update_service.rb
index 026a37997d4..30c5f24988c 100644
--- a/app/services/merge_requests/update_service.rb
+++ b/app/services/merge_requests/update_service.rb
@@ -16,7 +16,7 @@ module MergeRequests
       update(merge_request)
     end
 
-    def handle_changes(merge_request, old_labels: [])
+    def handle_changes(merge_request, old_labels: [], old_mentioned_users: [])
       if has_changes?(merge_request, old_labels: old_labels)
         todo_service.mark_pending_todos_as_done(merge_request, current_user)
       end
@@ -55,6 +55,15 @@ module MergeRequests
           current_user
         )
       end
+
+      added_mentions = merge_request.mentioned_users - old_mentioned_users
+      if added_mentions.present?
+        notification_service.new_mentions_in_merge_request(
+          merge_request,
+          added_mentions,
+          current_user
+        )
+      end
     end
 
     def reopen_service
diff --git a/app/services/notification_service.rb b/app/services/notification_service.rb
index ab6e51209ee..2291bc0f127 100644
--- a/app/services/notification_service.rb
+++ b/app/services/notification_service.rb
@@ -35,6 +35,20 @@ class NotificationService
     new_resource_email(issue, issue.project, :new_issue_email)
   end
 
+  # When issue text is updated, we should send an email to:
+  #
+  #  * newly mentioned project team members with notification level higher than Participating
+  #
+  def new_mentions_in_issue(issue, new_mentioned_users, current_user)
+    new_mentions_in_resource_email(
+      issue,
+      issue.project,
+      new_mentioned_users,
+      current_user,
+      :new_mention_in_issue_email
+    )
+  end
+
   # When we close an issue we should send an email to:
   #
   #  * issue author if their notification level is not Disabled
@@ -75,6 +89,20 @@ class NotificationService
     new_resource_email(merge_request, merge_request.target_project, :new_merge_request_email)
   end
 
+  # When merge request text is updated, we should send an email to:
+  #
+  #  * newly mentioned project team members with notification level higher than Participating
+  #
+  def new_mentions_in_merge_request(merge_request, new_mentioned_users, current_user)
+    new_mentions_in_resource_email(
+      merge_request,
+      merge_request.target_project,
+      new_mentioned_users,
+      current_user,
+      :new_mention_in_merge_request_email
+    )
+  end
+
   # When we reassign a merge_request we should send an email to:
   #
   #  * merge_request old assignee if their notification level is not Disabled
@@ -177,7 +205,7 @@ class NotificationService
 
     # build notify method like 'note_commit_email'
     notify_method = "note_#{note.noteable_type.underscore}_email".to_sym
-    
+
     recipients.each do |recipient|
       mailer.send(notify_method, recipient.id, note.id).deliver_later
     end
@@ -471,6 +499,15 @@ class NotificationService
     end
   end
 
+  def new_mentions_in_resource_email(target, project, new_mentioned_users, current_user, method)
+    recipients = build_recipients(target, project, current_user, action: "new")
+    recipients = recipients & new_mentioned_users
+
+    recipients.each do |recipient|
+      mailer.send(method, recipient.id, target.id, current_user.id).deliver_later
+    end
+  end
+
   def close_resource_email(target, project, current_user, method)
     action = method == :merged_merge_request_email ? "merge" : "close"
     recipients = build_recipients(target, project, current_user, action: action)
diff --git a/app/views/notify/new_mention_in_issue_email.html.haml b/app/views/notify/new_mention_in_issue_email.html.haml
new file mode 100644
index 00000000000..4f3d36bd9ca
--- /dev/null
+++ b/app/views/notify/new_mention_in_issue_email.html.haml
@@ -0,0 +1,12 @@
+%p
+  You have been mentioned in an issue.
+
+- if current_application_settings.email_author_in_body
+  %div
+    #{link_to @issue.author_name, user_url(@issue.author)} wrote:
+-if @issue.description
+  = markdown(@issue.description, pipeline: :email, author: @issue.author)
+
+- if @issue.assignee_id.present?
+  %p
+    Assignee: #{@issue.assignee_name}
diff --git a/app/views/notify/new_mention_in_issue_email.text.erb b/app/views/notify/new_mention_in_issue_email.text.erb
new file mode 100644
index 00000000000..457e94b4800
--- /dev/null
+++ b/app/views/notify/new_mention_in_issue_email.text.erb
@@ -0,0 +1,7 @@
+You have been mentioned in an issue.
+
+Issue <%= @issue.iid %>: <%= url_for(namespace_project_issue_url(@issue.project.namespace, @issue.project, @issue)) %>
+Author:    <%= @issue.author_name %>
+Assignee:  <%= @issue.assignee_name %>
+
+<%= @issue.description %>
diff --git a/app/views/notify/new_mention_in_merge_request_email.html.haml b/app/views/notify/new_mention_in_merge_request_email.html.haml
new file mode 100644
index 00000000000..32aedb9e6b9
--- /dev/null
+++ b/app/views/notify/new_mention_in_merge_request_email.html.haml
@@ -0,0 +1,15 @@
+%p
+  You have been mentioned in Merge Request #{@merge_request.to_reference}
+
+- if current_application_settings.email_author_in_body
+  %div
+    #{link_to @merge_request.author_name, user_url(@merge_request.author)} wrote:
+%p.details
+  != merge_path_description(@merge_request, '&rarr;')
+
+- if @merge_request.assignee_id.present?
+  %p
+    Assignee: #{@merge_request.author_name} &rarr; #{@merge_request.assignee_name}
+
+-if @merge_request.description
+  = markdown(@merge_request.description, pipeline: :email, author: @merge_request.author)
diff --git a/app/views/notify/new_mention_in_merge_request_email.text.erb b/app/views/notify/new_mention_in_merge_request_email.text.erb
new file mode 100644
index 00000000000..5bf0282e097
--- /dev/null
+++ b/app/views/notify/new_mention_in_merge_request_email.text.erb
@@ -0,0 +1,9 @@
+You have been mentioned in Merge Request <%= @merge_request.to_reference %>
+
+<%= url_for(namespace_project_merge_request_url(@merge_request.target_project.namespace, @merge_request.target_project, @merge_request)) %>
+
+<%= merge_path_description(@merge_request, 'to') %>
+Author:    <%= @merge_request.author_name %>
+Assignee:  <%= @merge_request.assignee_name %>
+
+<%= @merge_request.description %>
diff --git a/doc/workflow/notifications.md b/doc/workflow/notifications.md
index b4a9c2f3d3e..1b49a5c385f 100644
--- a/doc/workflow/notifications.md
+++ b/doc/workflow/notifications.md
@@ -67,7 +67,7 @@ In all of the below cases, the notification will be sent to:
 - Participants:
   - the author and assignee of the issue/merge request
   - authors of comments on the issue/merge request
-  - anyone mentioned by `@username` in the issue/merge request description
+  - anyone mentioned by `@username` in the issue/merge request title or description
   - anyone mentioned by `@username` in any of the comments on the issue/merge request
 
     ...with notification level "Participating" or higher
@@ -89,6 +89,11 @@ In all of the below cases, the notification will be sent to:
 | Merge merge request    | |
 | New comment            | The above, plus anyone mentioned by `@username` in the comment, with notification level "Mention" or higher |
 
+
+In addition, if the title or description of an Issue or Merge Request is
+changed, notifications will be sent to any **new** mentions by `@username` as
+if they had been mentioned in the original text.
+
 You won't receive notifications for Issues, Merge Requests or Milestones
 created by yourself. You will only receive automatic notifications when
 somebody else comments or adds changes to the ones that you've created or
diff --git a/spec/services/issues/update_service_spec.rb b/spec/services/issues/update_service_spec.rb
index 088c3d48bf7..0313f424463 100644
--- a/spec/services/issues/update_service_spec.rb
+++ b/spec/services/issues/update_service_spec.rb
@@ -319,5 +319,10 @@ describe Issues::UpdateService, services: true do
         end
       end
     end
+
+    context 'updating mentions' do
+      let(:mentionable) { issue }
+      include_examples 'updating mentions', Issues::UpdateService
+    end
   end
 end
diff --git a/spec/services/merge_requests/update_service_spec.rb b/spec/services/merge_requests/update_service_spec.rb
index 283a336afd9..6dfeb581975 100644
--- a/spec/services/merge_requests/update_service_spec.rb
+++ b/spec/services/merge_requests/update_service_spec.rb
@@ -226,6 +226,11 @@ describe MergeRequests::UpdateService, services: true do
       end
     end
 
+    context 'updating mentions' do
+      let(:mentionable) { merge_request }
+      include_examples 'updating mentions', MergeRequests::UpdateService
+    end
+
     context 'when MergeRequest has tasks' do
       before { update_merge_request({ description: "- [ ] Task 1\n- [ ] Task 2" }) }
 
diff --git a/spec/services/notification_service_spec.rb b/spec/services/notification_service_spec.rb
index 92b441c28ca..62c97e09288 100644
--- a/spec/services/notification_service_spec.rb
+++ b/spec/services/notification_service_spec.rb
@@ -9,6 +9,28 @@ describe NotificationService, services: true do
     end
   end
 
+  shared_examples 'notifications for new mentions' do
+    def send_notifications(*new_mentions)
+      reset_delivered_emails!
+      notification.send(notification_method, mentionable, new_mentions, @u_disabled)
+    end
+
+    it 'sends no emails when no new mentions are present' do
+      send_notifications
+      expect(ActionMailer::Base.deliveries).to be_empty
+    end
+
+    it 'emails new mentions with a watch level higher than participant' do
+      send_notifications(@u_watcher, @u_participant_mentioned, @u_custom_global)
+      should_only_email(@u_watcher, @u_participant_mentioned, @u_custom_global)
+    end
+
+    it 'does not email new mentions with a watch level equal to or less than participant' do
+      send_notifications(@u_participating, @u_mentioned)
+      expect(ActionMailer::Base.deliveries).to be_empty
+    end
+  end
+
   describe 'Keys' do
     describe '#new_key' do
       let!(:key) { create(:personal_key) }
@@ -399,6 +421,13 @@ describe NotificationService, services: true do
       end
     end
 
+    describe '#new_mentions_in_issue' do
+      let(:notification_method) { :new_mentions_in_issue }
+      let(:mentionable) { issue }
+
+      include_examples 'notifications for new mentions'
+    end
+
     describe '#reassigned_issue' do
       before do
         update_custom_notification(:reassign_issue, @u_guest_custom, project)
@@ -700,6 +729,8 @@ describe NotificationService, services: true do
     before do
       build_team(merge_request.target_project)
       add_users_with_subscription(merge_request.target_project, merge_request)
+      update_custom_notification(:new_merge_request, @u_guest_custom, project)
+      update_custom_notification(:new_merge_request, @u_custom_global)
       ActionMailer::Base.deliveries.clear
     end
 
@@ -763,6 +794,13 @@ describe NotificationService, services: true do
       end
     end
 
+    describe '#new_mentions_in_merge_request' do
+      let(:notification_method) { :new_mentions_in_merge_request }
+      let(:mentionable) { merge_request }
+
+      include_examples 'notifications for new mentions'
+    end
+
     describe '#reassigned_merge_request' do
       before do
         update_custom_notification(:reassign_merge_request, @u_guest_custom, project)
diff --git a/spec/support/email_helpers.rb b/spec/support/email_helpers.rb
index a85ab22ce36..0bfc4685532 100644
--- a/spec/support/email_helpers.rb
+++ b/spec/support/email_helpers.rb
@@ -3,6 +3,16 @@ module EmailHelpers
     ActionMailer::Base.deliveries.map(&:to).flatten.count(user.email) == 1
   end
 
+  def reset_delivered_emails!
+    ActionMailer::Base.deliveries.clear
+  end
+
+  def should_only_email(*users)
+    users.each {|user| should_email(user) }
+    recipients = ActionMailer::Base.deliveries.flat_map(&:to)
+    expect(recipients.count).to eq(users.count)
+  end
+
   def should_email(user)
     expect(sent_to_user?(user)).to be_truthy
   end
diff --git a/spec/support/updating_mentions_shared_examples.rb b/spec/support/updating_mentions_shared_examples.rb
new file mode 100644
index 00000000000..e0c59a5c280
--- /dev/null
+++ b/spec/support/updating_mentions_shared_examples.rb
@@ -0,0 +1,32 @@
+RSpec.shared_examples 'updating mentions' do |service_class|
+  let(:mentioned_user) { create(:user) }
+  let(:service_class) { service_class }
+
+  before { project.team << [mentioned_user, :developer] }
+
+  def update_mentionable(opts)
+    reset_delivered_emails!
+
+    perform_enqueued_jobs do
+      service_class.new(project, user, opts).execute(mentionable)
+    end
+
+    mentionable.reload
+  end
+
+  context 'in title' do
+    before { update_mentionable(title: mentioned_user.to_reference) }
+
+    it 'emails only the newly-mentioned user' do
+      should_only_email(mentioned_user)
+    end
+  end
+
+  context 'in description' do
+    before { update_mentionable(description: mentioned_user.to_reference) }
+
+    it 'emails only the newly-mentioned user' do
+      should_only_email(mentioned_user)
+    end
+  end
+end