diff --git a/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.html b/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.html
index 6d034b0..864294b 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.html
@@ -40,6 +40,7 @@
             project-config="[[projectConfig]]"
             on-reply="_handleCommentReply"
             on-comment-discard="_handleCommentDiscard"
+            on-ack="_handleCommentAck"
             on-done="_handleCommentDone"></gr-diff-comment>
       </template>
     </div>
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.js b/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.js
index a24100f..de1c197 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread.js
@@ -162,6 +162,18 @@
       this.push('comments', reply);
     },
 
+    _handleCommentAck: function(e) {
+      var comment = e.detail.comment;
+      var reply = this._newReply(comment.id, comment.line, 'Ack');
+      this.push('comments', reply);
+
+      // Allow the reply to render in the dom-repeat.
+      this.async(function() {
+        var commentEl = this._commentElWithDraftID(reply.__draftID);
+        commentEl.save();
+      }.bind(this), 1);
+    },
+
     _handleCommentDone: function(e) {
       var comment = e.detail.comment;
       var reply = this._newReply(comment.id, comment.line, 'Done');
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread_test.html b/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread_test.html
index 7b85b62..b694a75 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-comment-thread/gr-diff-comment-thread_test.html
@@ -180,6 +180,23 @@
           {bubbles: false});
     });
 
+    test('ack', function(done) {
+      element.changeNum = '42';
+      element.patchNum = '1';
+      var commentEl = element.$$('gr-diff-comment');
+      assert.ok(commentEl);
+      commentEl.addEventListener('ack', function() {
+        var drafts = element._orderedComments.filter(function(c) {
+          return c.__draft == true;
+        });
+        assert.equal(drafts.length, 1);
+        assert.equal(drafts[0].message, 'Ack');
+        assert.equal(drafts[0].in_reply_to, 'baf0414d_60047215');
+        done();
+      });
+      commentEl.fire('ack', {comment: commentEl.comment}, {bubbles: false});
+    });
+
     test('done', function(done) {
       element.changeNum = '42';
       element.patchNum = '1';
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.html b/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.html
index a7be3b7..a1c73db 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.html
@@ -93,11 +93,12 @@
       .danger .action {
         margin-right: 0;
       }
-      .container:not(.draft) .actions :not(.reply):not(.quote):not(.done) {
+      .container:not(.draft) .actions :not(.reply):not(.quote):not(.ack):not(.done) {
         display: none;
       }
       .draft .reply,
       .draft .quote,
+      .draft .ack,
       .draft .done {
         display: none;
       }
@@ -111,6 +112,7 @@
       .editing .message,
       .editing .reply,
       .editing .quote,
+      .editing .ack,
       .editing .done,
       .editing .edit {
         display: none;
@@ -191,6 +193,7 @@
       <div class="actions" hidden$="[[!showActions]]">
         <gr-button class="action reply" on-tap="_handleReply">Reply</gr-button>
         <gr-button class="action quote" on-tap="_handleQuote">Quote</gr-button>
+        <gr-button class="action ack" on-tap="_handleAck">Ack</gr-button>
         <gr-button class="action done" on-tap="_handleDone">Done</gr-button>
         <gr-button class="action edit" on-tap="_handleEdit">Edit</gr-button>
         <gr-button class="action save" on-tap="_handleSave"
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.js b/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.js
index 720e654..8905854 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.js
@@ -26,6 +26,12 @@
      */
 
     /**
+     * Fired when the Ack action is triggered.
+     *
+     * @event ack
+     */
+
+    /**
      * Fired when the Done action is triggered.
      *
      * @event done
@@ -289,6 +295,11 @@
           'reply', this._getEventPayload({quote: true}), {bubbles: false});
     },
 
+    _handleAck: function(e) {
+      e.preventDefault();
+      this.fire('ack', this._getEventPayload(), {bubbles: false});
+    },
+
     _handleDone: function(e) {
       e.preventDefault();
       this.fire('done', this._getEventPayload(), {bubbles: false});
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment_test.html b/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment_test.html
index 49507a5..96caebf 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment_test.html
@@ -115,6 +115,13 @@
       MockInteractions.tap(element.$$('.quote'));
     });
 
+    test('proper event fires on ack', function(done) {
+      element.addEventListener('ack', function(e) {
+        done();
+      });
+      MockInteractions.tap(element.$$('.ack'));
+    });
+
     test('proper event fires on done', function(done) {
       element.addEventListener('done', function(e) {
         done();
@@ -227,6 +234,7 @@
       assert.isFalse(isVisible(element.$$('.cancel')), 'cancel is not visible');
       assert.isFalse(isVisible(element.$$('.reply')), 'reply is not visible');
       assert.isFalse(isVisible(element.$$('.quote')), 'quote is not visible');
+      assert.isFalse(isVisible(element.$$('.ack')), 'ack is not visible');
       assert.isFalse(isVisible(element.$$('.done')), 'done is not visible');
 
       element.editing = true;
@@ -236,6 +244,7 @@
       assert.isFalse(isVisible(element.$$('.cancel')), 'cancel is visible');
       assert.isFalse(isVisible(element.$$('.reply')), 'reply is not visible');
       assert.isFalse(isVisible(element.$$('.quote')), 'quote is not visible');
+      assert.isFalse(isVisible(element.$$('.ack')), 'ack is not visible');
       assert.isFalse(isVisible(element.$$('.done')), 'done is not visible');
 
       element.draft = false;
@@ -247,6 +256,7 @@
       assert.isFalse(isVisible(element.$$('.cancel')), 'cancel is not visible');
       assert.isTrue(isVisible(element.$$('.reply')), 'reply is visible');
       assert.isTrue(isVisible(element.$$('.quote')), 'quote is visible');
+      assert.isTrue(isVisible(element.$$('.ack')), 'ack is visible');
       assert.isTrue(isVisible(element.$$('.done')), 'done is visible');
 
       element.comment.id = 'foo';