diff --git a/app/notification.go b/app/notification.go
index 6c6fc78f73..d040e4d33c 100644
--- a/app/notification.go
+++ b/app/notification.go
@@ -100,7 +100,6 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 		keywords := a.getMentionKeywordsInChannel(profileMap, allowChannelMentions, channelMemberNotifyPropsMap)
 
 		mentions = getExplicitMentions(post, keywords, groups)
-
 		// Add an implicit mention when a user is added to a channel
 		// even if the user has set 'username mentions' to false in account settings.
 		if post.Type == model.POST_ADD_TO_CHANNEL {
@@ -124,6 +123,8 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 
 		// get users that have comment thread mentions enabled
 		if post.RootId != "" && parentPostList != nil {
+			rootPost := parentPostList.Posts[parentPostList.Order[0]]
+			mentions.merge(getExplicitMentions(rootPost, keywords, groups))
 			for _, threadPost := range parentPostList.Posts {
 				profile := profileMap[threadPost.UserId]
 				if profile == nil {
@@ -756,6 +757,12 @@ func (m *ExplicitMentions) addMentions(userIDs []string, mentionType MentionType
 	}
 }
 
+func (m *ExplicitMentions) merge(other *ExplicitMentions) {
+	for userID, mentionType := range other.Mentions {
+		m.addMention(userID, mentionType)
+	}
+}
+
 func (m *ExplicitMentions) removeMention(userID string) {
 	delete(m.Mentions, userID)
 }
diff --git a/app/post_test.go b/app/post_test.go
index 269624e68f..f28a49b4f6 100644
--- a/app/post_test.go
+++ b/app/post_test.go
@@ -1924,6 +1924,36 @@ func TestThreadMembership(t *testing.T) {
 	})
 }
 
+func TestAutofollowBasedOnRootPost(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+	os.Setenv("MM_FEATUREFLAGS_COLLAPSEDTHREADS", "true")
+	defer os.Unsetenv("MM_FEATUREFLAGS_COLLAPSEDTHREADS")
+
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		*cfg.ServiceSettings.ThreadAutoFollow = true
+		*cfg.ServiceSettings.CollapsedThreads = model.COLLAPSED_THREADS_DEFAULT_ON
+	})
+
+	channel := th.BasicChannel
+	user := th.BasicUser
+	user2 := th.BasicUser2
+	appErr := th.App.JoinChannel(channel, user.Id)
+	require.Nil(t, appErr)
+	appErr = th.App.JoinChannel(channel, user2.Id)
+	require.Nil(t, appErr)
+	p1, err := th.App.CreatePost(&model.Post{UserId: user.Id, ChannelId: channel.Id, Message: "Hi @" + user2.Username}, channel, false, false)
+	require.Nil(t, err)
+	m, e := th.App.GetThreadMembershipsForUser(user2.Id, th.BasicTeam.Id)
+	require.NoError(t, e)
+	require.Len(t, m, 0)
+	_, err2 := th.App.CreatePost(&model.Post{RootId: p1.Id, UserId: user.Id, ChannelId: channel.Id, Message: "Hola"}, channel, false, false)
+	require.Nil(t, err2)
+	m, e = th.App.GetThreadMembershipsForUser(user2.Id, th.BasicTeam.Id)
+	require.NoError(t, e)
+	require.Len(t, m, 1)
+}
+
 func TestCollapsedThreadFetch(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()