diff --git a/addons/mail/static/src/js/discuss.js b/addons/mail/static/src/js/discuss.js
index e93adeaae30..2cb9fe749d7 100644
--- a/addons/mail/static/src/js/discuss.js
+++ b/addons/mail/static/src/js/discuss.js
@@ -1321,12 +1321,16 @@ var Discuss = AbstractAction.extend(ControlPanelMixin, {
      */
     _onPostMessage: function (messageData) {
         var self = this;
+        var options = {};
         if (this._selectedMessage) {
             messageData.subtype = this._selectedMessage.isNote() ? 'mail.mt_note': 'mail.mt_comment';
             messageData.subtype_id = false;
             messageData.message_type = 'comment';
+
+            options.documentID = this._selectedMessage.getDocumentID();
+            options.documentModel = this._selectedMessage.getDocumentModel();
         }
-        this._thread.postMessage(messageData)
+        this._thread.postMessage(messageData, options)
             .then(function () {
                 if (self._selectedMessage) {
                     self._renderSnackbar('mail.discuss.MessageSentSnackbar', {
diff --git a/addons/mail/static/src/js/models/threads/mailbox.js b/addons/mail/static/src/js/models/threads/mailbox.js
index 82d65bdd6cc..263072f575e 100644
--- a/addons/mail/static/src/js/models/threads/mailbox.js
+++ b/addons/mail/static/src/js/models/threads/mailbox.js
@@ -189,6 +189,27 @@ var Mailbox = SearchableThread.extend({
             throw (_.str(_t("Missing domain for mailbox with ID '%s'"), this._id));
         }
     },
+    /**
+     * Post a message from inbox. This is used when using the 'reply' feature
+     * on a message that is linked to a document thread.
+     *
+     * @override
+     * @private
+     * @param {Object} messageData
+     * @param {Object} options
+     * @param {integer} options.documentID
+     * @param {string} options.documentModel
+     * @returns {$.Promise}
+     */
+    _postMessage: function (messageData, options) {
+        var documentThread = this.call(
+            'mail_service',
+            'getDocumentThread',
+            options.documentModel,
+            options.documentID
+        );
+        return documentThread.postMessage(messageData);
+    },
 });
 
 return Mailbox;
diff --git a/addons/mail/static/src/xml/thread.xml b/addons/mail/static/src/xml/thread.xml
index e5db4a72524..f28e071b2f3 100644
--- a/addons/mail/static/src/xml/thread.xml
+++ b/addons/mail/static/src/xml/thread.xml
@@ -338,7 +338,7 @@
                         <i t-if="options.displayStars and !message.isSystemNotification() and !message.needsModeration()"
                             t-att-class="'fa fa-lg o_thread_icon o_thread_message_star ' + (message.isStarred() ? 'fa-star' : 'fa-star-o')"
                             t-att-data-message-id="message.getID()" title="Mark as Todo" role="img" aria-label="Mark as Todo"/>
-                       <i t-if="message.originatesFromChannel() and options.displayReplyIcons"
+                       <i t-if="message.isLinkedToDocumentThread() and options.displayReplyIcons"
                            class="fa fa-reply o_thread_icon o_thread_message_reply"
                            t-att-data-message-id="message.getID()" title="Reply" role="img" aria-label="Reply"/>
                         <i t-if="message.isNeedaction() and options.displayMarkAsRead"
diff --git a/addons/mail/static/tests/discuss_tests.js b/addons/mail/static/tests/discuss_tests.js
index e22d24f5002..9913435363c 100644
--- a/addons/mail/static/tests/discuss_tests.js
+++ b/addons/mail/static/tests/discuss_tests.js
@@ -1068,5 +1068,84 @@ QUnit.test('mark all messages as read from Inbox', function (assert) {
     });
 });
 
+QUnit.test('reply to message from inbox', function (assert) {
+    var done = assert.async();
+    assert.expect(11);
+
+    var self = this;
+    this.data['mail.message'].records = [{
+        author_id: [5, 'Demo User'],
+        body: '<p>test 1</p>',
+        id: 1,
+        needaction: true,
+        needaction_partner_ids: [3],
+        res_id: 100,
+        model: 'some.document',
+        record_name: 'SomeDocument',
+    }];
+    this.data.initMessaging = {
+        needaction_inbox_counter: 1,
+    };
+
+    createDiscuss({
+        id: 1,
+        context: {},
+        params: {},
+        data: this.data,
+        services: this.services,
+        session: { partner_id: 3 },
+        mockRPC: function (route, args) {
+            if (args.method === 'message_post') {
+                assert.step(args.method);
+                assert.strictEqual(args.model, 'some.document',
+                    "should post message to correct document model");
+                assert.strictEqual(args.args[0], 100,
+                    "should post message to correct document ID");
+
+                self.data['mail.message'].records.push({
+                    author_id: [3, 'Me'],
+                    body: args.kwargs.body,
+                    id: 2,
+                    res_id: 100,
+                    model: 'some.document',
+                    record_name: 'SomeDocument',
+                });
+                return $.when(2);
+            }
+            return this._super.apply(this, arguments);
+        },
+    })
+    .then(function (discuss) {
+        assert.strictEqual(discuss.$('.o_mail_discuss_item.o_active').data('thread-id'),
+            'mailbox_inbox',
+            "Inbox should be selected by default");
+        assert.containsOnce(discuss, '.o_thread_message',
+            "should display a single message in inbox");
+        assert.strictEqual(discuss.$('.o_thread_message').data('message-id'), 1,
+            "message should be linked to correct message");
+        assert.containsOnce(discuss.$('.o_thread_message'), '.o_thread_message_reply',
+            "should display the reply icon for message linked to a document");
+
+        testUtils.dom.click(discuss.$('.o_thread_message_reply'));
+        var $composer = discuss.$('.o_thread_composer_extended');
+        assert.isVisible($composer,
+            "extended composer should become visible");
+        assert.strictEqual($composer.find('.o_composer_subject input').val(),
+            'Re: SomeDocument',
+            "composer should have copied document name as subject of message");
+
+        var $textarea = $composer.find('.o_composer_input textarea').first();
+        testUtils.fields.editInput($textarea, 'someContent');
+        assert.containsOnce($composer, '.o_composer_button_send',
+            "should have button to send reply message");
+        testUtils.dom.click($composer.find('.o_composer_button_send'));
+
+        assert.verifySteps(['message_post']);
+
+        discuss.destroy();
+        done();
+    });
+});
+
 });
 });