diff --git a/api4/channel.go b/api4/channel.go
index 3aaa96aafe..5bfbddf460 100644
--- a/api4/channel.go
+++ b/api4/channel.go
@@ -1698,6 +1698,14 @@ func addChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if postRootId != "" {
+		err := c.App.UpdateThreadFollowForUserFromChannelAdd(cm.UserId, channel.TeamId, postRootId)
+		if err != nil {
+			c.Err = err
+			return
+		}
+	}
+
 	auditRec.Success()
 	auditRec.AddMeta("add_user_id", cm.UserId)
 	c.LogAudit("name=" + channel.Name + " user_id=" + cm.UserId)
diff --git a/api4/channel_test.go b/api4/channel_test.go
index 5741615bf5..45f140a7bc 100644
--- a/api4/channel_test.go
+++ b/api4/channel_test.go
@@ -3012,6 +3012,63 @@ func TestAddChannelMember(t *testing.T) {
 	})
 }
 
+func TestAddChannelMemberFromThread(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+	team := th.BasicTeam
+	user := th.BasicUser
+	user2 := th.BasicUser2
+	user3 := th.CreateUserWithClient(th.SystemAdminClient)
+	_, _, err := th.SystemAdminClient.AddTeamMember(team.Id, user3.Id)
+	require.NoError(t, err)
+
+	publicChannel := th.CreatePublicChannel()
+
+	_, resp, err := th.Client.AddChannelMember(publicChannel.Id, user.Id)
+	require.NoError(t, err)
+	CheckCreatedStatus(t, resp)
+	_, resp, err = th.Client.AddChannelMember(publicChannel.Id, user2.Id)
+	require.NoError(t, err)
+	CheckCreatedStatus(t, resp)
+
+	post := &model.Post{
+		ChannelId: publicChannel.Id,
+		Message:   "A root post",
+		UserId:    user.Id,
+	}
+	rpost, _, err := th.SystemAdminClient.CreatePost(post)
+	require.NoError(t, err)
+
+	_, _, err = th.SystemAdminClient.CreatePost(
+		&model.Post{
+			ChannelId: publicChannel.Id,
+			Message:   "A reply post with mention @" + user3.Username,
+			UserId:    user2.Id,
+			RootId:    rpost.Id,
+		})
+	require.NoError(t, err)
+
+	_, _, err = th.SystemAdminClient.CreatePost(
+		&model.Post{
+			ChannelId: publicChannel.Id,
+			Message:   "Another reply post with mention @" + user3.Username,
+			UserId:    user2.Id,
+			RootId:    rpost.Id,
+		})
+	require.NoError(t, err)
+
+	// Simulate adding a user to a channel from a thread
+	_, _, err = th.SystemAdminClient.AddChannelMemberWithRootId(publicChannel.Id, user3.Id, rpost.Id)
+	require.NoError(t, err)
+
+	// Threadmembership should exist for added user
+	ut, _, err := th.SystemAdminClient.GetUserThread(user3.Id, team.Id, rpost.Id, false)
+	require.NoError(t, err)
+	// Should have two mentions. There might be a race condition
+	// here between the "added user to the channel" message and the GetUserThread call
+	require.LessOrEqual(t, int64(2), ut.UnreadMentions)
+}
+
 func TestAddChannelMemberAddMyself(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
diff --git a/app/app_iface.go b/app/app_iface.go
index 108934b0f8..272358df7d 100644
--- a/app/app_iface.go
+++ b/app/app_iface.go
@@ -1063,6 +1063,7 @@ type AppIface interface {
 	UpdateTeamPrivacy(teamID string, teamType string, allowOpenInvite bool) *model.AppError
 	UpdateTeamScheme(team *model.Team) (*model.Team, *model.AppError)
 	UpdateThreadFollowForUser(userID, teamID, threadID string, state bool) *model.AppError
+	UpdateThreadFollowForUserFromChannelAdd(userID, teamID, threadID string) *model.AppError
 	UpdateThreadReadForUser(currentSessionId, userID, teamID, threadID string, timestamp int64) (*model.ThreadResponse, *model.AppError)
 	UpdateThreadsReadForUser(userID, teamID string) *model.AppError
 	UpdateUser(user *model.User, sendNotifications bool) (*model.User, *model.AppError)
diff --git a/app/opentracing/opentracing_layer.go b/app/opentracing/opentracing_layer.go
index 695bce2a25..367a6af60b 100644
--- a/app/opentracing/opentracing_layer.go
+++ b/app/opentracing/opentracing_layer.go
@@ -16634,6 +16634,28 @@ func (a *OpenTracingAppLayer) UpdateThreadFollowForUser(userID string, teamID st
 	return resultVar0
 }
 
+func (a *OpenTracingAppLayer) UpdateThreadFollowForUserFromChannelAdd(userID string, teamID string, threadID string) *model.AppError {
+	origCtx := a.ctx
+	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.UpdateThreadFollowForUserFromChannelAdd")
+
+	a.ctx = newCtx
+	a.app.Srv().Store.SetContext(newCtx)
+	defer func() {
+		a.app.Srv().Store.SetContext(origCtx)
+		a.ctx = origCtx
+	}()
+
+	defer span.Finish()
+	resultVar0 := a.app.UpdateThreadFollowForUserFromChannelAdd(userID, teamID, threadID)
+
+	if resultVar0 != nil {
+		span.LogFields(spanlog.Error(resultVar0))
+		ext.Error.Set(span, true)
+	}
+
+	return resultVar0
+}
+
 func (a *OpenTracingAppLayer) UpdateThreadReadForUser(currentSessionId string, userID string, teamID string, threadID string, timestamp int64) (*model.ThreadResponse, *model.AppError) {
 	origCtx := a.ctx
 	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.UpdateThreadReadForUser")
diff --git a/app/user.go b/app/user.go
index 34dab0b6dd..1cb5396234 100644
--- a/app/user.go
+++ b/app/user.go
@@ -2301,6 +2301,64 @@ func (a *App) UpdateThreadFollowForUser(userID, teamID, threadID string, state b
 	return nil
 }
 
+func (a *App) UpdateThreadFollowForUserFromChannelAdd(userID, teamID, threadID string) *model.AppError {
+	opts := store.ThreadMembershipOpts{
+		Following:             true,
+		IncrementMentions:     false,
+		UpdateFollowing:       true,
+		UpdateViewedTimestamp: false,
+		UpdateParticipants:    false,
+	}
+	tm, err := a.Srv().Store.Thread().MaintainMembership(userID, threadID, opts)
+	if err != nil {
+		return model.NewAppError("UpdateThreadFollowForUserFromChannelAdd", "app.user.update_thread_follow_for_user.app_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+
+	post, appErr := a.GetSinglePost(threadID)
+	if appErr != nil {
+		return appErr
+	}
+	user, appErr := a.GetUser(userID)
+	if appErr != nil {
+		return appErr
+	}
+	tm.UnreadMentions, appErr = a.countThreadMentions(user, post, teamID, post.CreateAt-1)
+	if appErr != nil {
+		return appErr
+	}
+	tm.LastViewed = post.CreateAt - 1
+	_, err = a.Srv().Store.Thread().UpdateMembership(tm)
+	if err != nil {
+		return model.NewAppError("UpdateThreadFollowForUserFromChannelAdd", "app.user.update_thread_follow_for_user.app_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+
+	message := model.NewWebSocketEvent(model.WebsocketEventThreadUpdated, teamID, "", userID, nil)
+	userThread, err := a.Srv().Store.Thread().GetThreadForUser(teamID, tm, true)
+	if err != nil {
+		var errNotFound *store.ErrNotFound
+		if errors.As(err, &errNotFound) {
+			return nil
+		}
+		return model.NewAppError("UpdateThreadFollowForUserFromChannelAdd", "app.user.update_thread_follow_for_user.app_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+	a.sanitizeProfiles(userThread.Participants, false)
+	userThread.Post.SanitizeProps()
+	sanitizedPost, appErr := a.SanitizePostMetadataForUser(userThread.Post, userID)
+	if appErr != nil {
+		return appErr
+	}
+	userThread.Post = sanitizedPost
+
+	payload, jsonErr := json.Marshal(userThread)
+	if jsonErr != nil {
+		mlog.Warn("Failed to encode thread to JSON")
+	}
+	message.Add("thread", string(payload))
+
+	a.Publish(message)
+	return nil
+}
+
 func (a *App) UpdateThreadReadForUser(currentSessionId, userID, teamID, threadID string, timestamp int64) (*model.ThreadResponse, *model.AppError) {
 	user, err := a.GetUser(userID)
 	if err != nil {
diff --git a/store/sqlstore/thread_store.go b/store/sqlstore/thread_store.go
index 94d0aaaa8f..2ccce96e4b 100644
--- a/store/sqlstore/thread_store.go
+++ b/store/sqlstore/thread_store.go
@@ -425,6 +425,9 @@ func (s *SqlThreadStore) GetThreadForUser(teamId string, threadMembership *model
 
 	err := s.GetReplica().SelectOne(&thread, query, args...)
 	if err != nil {
+		if err == sql.ErrNoRows {
+			return nil, store.NewErrNotFound("Thread", threadMembership.PostId)
+		}
 		return nil, err
 	}