diff --git a/server/channels/api4/post_test.go b/server/channels/api4/post_test.go
index 31522e02d75..77e77931d5b 100644
--- a/server/channels/api4/post_test.go
+++ b/server/channels/api4/post_test.go
@@ -5329,6 +5329,32 @@ func TestGetPostStripActionIntegrations(t *testing.T) {
 	require.Nil(t, action["integration"])
 }
 
+func waitForPostReminderEphemeral(t *testing.T, userWSClient *model.WebSocketClient, timeout time.Duration) *model.Post {
+	t.Helper()
+	timer := time.NewTimer(timeout)
+	defer timer.Stop()
+	for {
+		select {
+		case ev := <-userWSClient.EventChannel:
+			if ev.EventType() != model.WebsocketEventEphemeralMessage {
+				continue
+			}
+			data := ev.GetData()
+			postJSON, ok := data["post"].(string)
+			require.True(t, ok)
+			var parsedPost model.Post
+			err := json.Unmarshal([]byte(postJSON), &parsedPost)
+			require.NoError(t, err)
+			if parsedPost.GetProp("type") != model.PostTypeReminder {
+				continue
+			}
+			return &parsedPost
+		case <-timer.C:
+			require.Fail(t, "timed out waiting for post reminder ephemeral websocket event")
+			return nil
+		}
+	}
+}
 func TestPostReminder(t *testing.T) {
 	th := Setup(t).InitBasic(t)
 
@@ -5352,39 +5378,53 @@ func TestPostReminder(t *testing.T) {
 	user, _, err := client.GetUser(context.Background(), post.UserId, "")
 	require.NoError(t, err)
 
-	var caught bool
-	func() {
-		for {
-			select {
-			case ev := <-userWSClient.EventChannel:
-				if ev.EventType() == model.WebsocketEventEphemeralMessage {
-					caught = true
-					data := ev.GetData()
+	parsedPost := waitForPostReminderEphemeral(t, userWSClient, 15*time.Second)
+	require.NotNil(t, parsedPost)
 
-					post, ok := data["post"].(string)
-					require.True(t, ok)
+	assert.Equal(t, model.PostTypeEphemeral, parsedPost.Type)
+	assert.Equal(t, th.BasicUser.Id, parsedPost.UserId)
+	assert.Equal(t, th.BasicPost.Id, parsedPost.RootId)
 
-					var parsedPost model.Post
-					err := json.Unmarshal([]byte(post), &parsedPost)
-					require.NoError(t, err)
+	require.Equal(t, float64(targetTime), parsedPost.GetProp("target_time").(float64))
+	require.Equal(t, th.BasicPost.Id, parsedPost.GetProp("post_id").(string))
+	require.Equal(t, user.Username, parsedPost.GetProp("username").(string))
+	require.Equal(t, th.BasicTeam.Name, parsedPost.GetProp("team_name").(string))
+}
 
-					assert.Equal(t, model.PostTypeEphemeral, parsedPost.Type)
-					assert.Equal(t, th.BasicUser.Id, parsedPost.UserId)
-					assert.Equal(t, th.BasicPost.Id, parsedPost.RootId)
+func TestPostReminderReplyEphemeralUsesThreadRoot(t *testing.T) {
+	th := Setup(t).InitBasic(t)
+	client := th.Client
+	userWSClient := th.CreateConnectedWebSocketClient(t)
 
-					require.Equal(t, float64(targetTime), parsedPost.GetProp("target_time").(float64))
-					require.Equal(t, th.BasicPost.Id, parsedPost.GetProp("post_id").(string))
-					require.Equal(t, user.Username, parsedPost.GetProp("username").(string))
-					require.Equal(t, th.BasicTeam.Name, parsedPost.GetProp("team_name").(string))
-					return
-				}
-			case <-time.After(15 * time.Second):
-				return
-			}
-		}
-	}()
+	replyPost := &model.Post{
+		ChannelId: th.BasicChannel.Id,
+		RootId:    th.BasicPost.Id,
+		Message:   "reply for reminder " + model.NewId(),
+	}
+	replyCreated, _, err := client.CreatePost(context.Background(), replyPost)
+	require.NoError(t, err)
 
-	require.Truef(t, caught, "User should have received %s event", model.WebsocketEventEphemeralMessage)
+	targetTime := time.Now().UTC().Unix() + 1
+	resp, err := client.SetPostReminder(context.Background(), &model.PostReminder{
+		TargetTime: targetTime,
+		PostId:     replyCreated.Id,
+		UserId:     th.BasicUser.Id,
+	})
+	require.NoError(t, err)
+	CheckOKStatus(t, resp)
+
+	user, _, err := client.GetUser(context.Background(), replyCreated.UserId, "")
+	require.NoError(t, err)
+
+	parsedPost := waitForPostReminderEphemeral(t, userWSClient, 15*time.Second)
+	require.NotNil(t, parsedPost)
+
+	assert.Equal(t, model.PostTypeEphemeral, parsedPost.Type)
+	assert.Equal(t, th.BasicPost.Id, parsedPost.RootId, "ephemeral should be threaded under the thread root so RHS/center thread views show the ack")
+	assert.Equal(t, replyCreated.Id, parsedPost.GetProp("post_id").(string))
+	require.Equal(t, float64(targetTime), parsedPost.GetProp("target_time").(float64))
+	require.Equal(t, user.Username, parsedPost.GetProp("username").(string))
+	require.Equal(t, th.BasicTeam.Name, parsedPost.GetProp("team_name").(string))
 }
 
 func TestPostGetInfo(t *testing.T) {
diff --git a/server/channels/app/post.go b/server/channels/app/post.go
index 1ea06c5fc55..3c92420a32c 100644
--- a/server/channels/app/post.go
+++ b/server/channels/app/post.go
@@ -2678,18 +2678,27 @@ func (a *App) populateEditHistoryFileMetadata(editHistoryPosts []*model.Post) *m
 }
 
 func (a *App) SetPostReminder(rctx request.CTX, postID, userID string, targetTime int64) *model.AppError {
-	// Store the reminder in the DB
+	remindedPost, postErr := a.GetSinglePost(rctx, postID, false)
+	if postErr != nil {
+		return postErr
+	}
+	// Ephemeral ack must use the thread root so CRT/RHS thread views (keyed by root id) show the confirmation.
+	ephemeralRootID := remindedPost.Id
+	if remindedPost.RootId != "" {
+		ephemeralRootID = remindedPost.RootId
+	}
+
+	metadata, err := a.Srv().Store().Post().GetPostReminderMetadata(postID)
+	if err != nil {
+		return model.NewAppError("SetPostReminder", model.NoTranslation, nil, "", http.StatusInternalServerError).Wrap(err)
+	}
+
 	reminder := &model.PostReminder{
 		PostId:     postID,
 		UserId:     userID,
 		TargetTime: targetTime,
 	}
-	err := a.Srv().Store().Post().SetPostReminder(reminder)
-	if err != nil {
-		return model.NewAppError("SetPostReminder", model.NoTranslation, nil, "", http.StatusInternalServerError).Wrap(err)
-	}
-
-	metadata, err := a.Srv().Store().Post().GetPostReminderMetadata(postID)
+	err = a.Srv().Store().Post().SetPostReminder(reminder)
 	if err != nil {
 		return model.NewAppError("SetPostReminder", model.NoTranslation, nil, "", http.StatusInternalServerError).Wrap(err)
 	}
@@ -2710,7 +2719,7 @@ func (a *App) SetPostReminder(rctx request.CTX, postID, userID string, targetTim
 		Id:        model.NewId(),
 		CreateAt:  model.GetMillis(),
 		UserId:    userID,
-		RootId:    postID,
+		RootId:    ephemeralRootID,
 		ChannelId: metadata.ChannelID,
 		// It's okay to keep this non-translated. This is just a fallback.
 		// The webapp will parse the timestamp and show that in user's local timezone.