diff --git a/api4/user_test.go b/api4/user_test.go
index 0afab99d12..75aec717e1 100644
--- a/api4/user_test.go
+++ b/api4/user_test.go
@@ -6116,6 +6116,50 @@ func TestThreadSocketEvents(t *testing.T) {
 			require.Truef(t, caught, "User should have received %s event", model.WebsocketEventThreadUpdated)
 		}
 	})
+
+	t.Run("Listen for thread updated event after create post when not previously following the thread", func(t *testing.T) {
+		rpost2 := &model.Post{ChannelId: th.BasicChannel.Id, UserId: th.BasicUser2.Id, Message: "root post"}
+
+		var appErr *model.AppError
+		rpost2, appErr = th.App.CreatePostAsUser(th.Context, rpost2, th.Context.Session().Id, false)
+		require.Nil(t, appErr)
+
+		reply1 := &model.Post{ChannelId: th.BasicChannel.Id, UserId: th.BasicUser2.Id, Message: "reply 1", RootId: rpost2.Id}
+		reply2 := &model.Post{ChannelId: th.BasicChannel.Id, UserId: th.BasicUser2.Id, Message: "reply 2", RootId: rpost2.Id}
+		reply3 := &model.Post{ChannelId: th.BasicChannel.Id, UserId: th.BasicUser2.Id, Message: "mention @" + th.BasicUser.Username, RootId: rpost2.Id}
+
+		_, appErr = th.App.CreatePostAsUser(th.Context, reply1, th.Context.Session().Id, false)
+		require.Nil(t, appErr)
+		_, appErr = th.App.CreatePostAsUser(th.Context, reply2, th.Context.Session().Id, false)
+		require.Nil(t, appErr)
+		_, appErr = th.App.CreatePostAsUser(th.Context, reply3, th.Context.Session().Id, false)
+		require.Nil(t, appErr)
+
+		count := 0
+		func() {
+			for {
+				select {
+				case ev := <-userWSClient.EventChannel:
+					if ev.EventType() == model.WebsocketEventThreadUpdated {
+						count++
+						data := ev.GetData()
+						var thread model.ThreadResponse
+						jsonErr := json.Unmarshal([]byte(data["thread"].(string)), &thread)
+						require.NoError(t, jsonErr)
+
+						require.Equal(t, int64(0), int64(data["previous_unread_replies"].(float64)))
+						require.Equal(t, int64(0), int64(data["previous_unread_mentions"].(float64)))
+						require.Equal(t, int64(3), thread.UnreadReplies)
+						require.Equal(t, int64(1), thread.UnreadMentions)
+					}
+				case <-time.After(1 * time.Second):
+					return
+				}
+			}
+		}()
+
+		require.Equalf(t, 1, count, "User should have received 1 %s event", model.WebsocketEventThreadUpdated)
+	})
 }
 
 func TestFollowThreads(t *testing.T) {
diff --git a/app/notification.go b/app/notification.go
index b1385cbd4c..7e46290e62 100644
--- a/app/notification.go
+++ b/app/notification.go
@@ -198,6 +198,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 	mentionedUsersList := make(model.StringArray, 0, len(mentions.Mentions))
 	mentionAutofollowChans := []chan *model.AppError{}
 	threadParticipants := map[string]bool{post.UserId: true}
+	newParticipants := map[string]bool{}
 	participantMemberships := map[string]*model.ThreadMembership{}
 	membershipsMutex := &sync.Mutex{}
 	followersMutex := &sync.Mutex{}
@@ -246,9 +247,6 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 					}
 
 					if membership != nil && !membership.Following {
-						membershipsMutex.Lock()
-						participantMemberships[userID] = membership
-						membershipsMutex.Unlock()
 						return
 					}
 				}
@@ -275,6 +273,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 				// add new followers to existing followers
 				if threadMembership.Following && !followers.Contains(userID) {
 					followers = append(followers, userID)
+					newParticipants[userID] = true
 				}
 				followersMutex.Unlock()
 
@@ -595,11 +594,19 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 					return nil, errors.Wrapf(err, "cannot get thread %q for user %q", post.RootId, uid)
 				}
 				if userThread != nil {
-					previousUnreadMentions := userThread.UnreadMentions
-					previousUnreadReplies := max(userThread.UnreadReplies-1, 0)
-					if mentions.isUserMentioned(uid) {
-						previousUnreadMentions = max(userThread.UnreadMentions-1, 0)
+					previousUnreadMentions := int64(0)
+					previousUnreadReplies := int64(0)
+
+					// if it's not a newly followed thread, calculate previous unread values.
+					if !newParticipants[uid] {
+						previousUnreadMentions = userThread.UnreadMentions
+						previousUnreadReplies = max(userThread.UnreadReplies-1, 0)
+
+						if mentions.isUserMentioned(uid) {
+							previousUnreadMentions = max(userThread.UnreadMentions-1, 0)
+						}
 					}
+
 					// set LastViewed to now for commenter
 					if uid == post.UserId {
 						opts := store.ThreadMembershipOpts{