diff --git a/api4/user.go b/api4/user.go
index 4b05ad3687..0166efc656 100644
--- a/api4/user.go
+++ b/api4/user.go
@@ -2939,7 +2939,7 @@ func getThreadsForUser(c *Context, w http.ResponseWriter, r *http.Request) {
 		Since:    0,
 		Before:   "",
 		After:    "",
-		PageSize: 30,
+		PageSize: uint64(c.Params.PerPage),
 		Unread:   false,
 		Extended: false,
 		Deleted:  false,
@@ -2962,15 +2962,6 @@ func getThreadsForUser(c *Context, w http.ResponseWriter, r *http.Request) {
 		c.Err = model.NewAppError("api.getThreadsForUser", "api.getThreadsForUser.bad_params", nil, "", http.StatusBadRequest)
 		return
 	}
-	pageSizeString := r.URL.Query().Get("pageSize")
-	if pageSizeString != "" {
-		pageSize, parseError := strconv.ParseUint(pageSizeString, 10, 64)
-		if parseError != nil {
-			c.SetInvalidParam("pageSize")
-			return
-		}
-		options.PageSize = pageSize
-	}
 
 	deletedStr := r.URL.Query().Get("deleted")
 	unreadStr := r.URL.Query().Get("unread")
diff --git a/api4/user_test.go b/api4/user_test.go
index 785bb49373..fe6730c3be 100644
--- a/api4/user_test.go
+++ b/api4/user_test.go
@@ -5621,7 +5621,8 @@ func TestGetThreadsForUser(t *testing.T) {
 		defer th.App.Srv().Store.Post().PermanentDeleteByUser(th.BasicUser.Id)
 
 		uss, _, err := th.Client.GetUserThreads(th.BasicUser.Id, th.BasicTeam.Id, model.GetUserThreadsOpts{
-			Deleted: false,
+			Deleted:  false,
+			PageSize: 30,
 		})
 		require.NoError(t, err)
 		require.Len(t, uss.Threads, 30)
diff --git a/model/client4.go b/model/client4.go
index 4d6fe1f2c5..afe48000b8 100644
--- a/model/client4.go
+++ b/model/client4.go
@@ -6593,7 +6593,7 @@ func (c *Client4) GetUserThreads(userId, teamId string, options GetUserThreadsOp
 		v.Set("after", options.After)
 	}
 	if options.PageSize != 0 {
-		v.Set("pageSize", fmt.Sprintf("%d", options.PageSize))
+		v.Set("per_page", fmt.Sprintf("%d", options.PageSize))
 	}
 	if options.Extended {
 		v.Set("extended", "true")
diff --git a/web/params.go b/web/params.go
index bec2d26693..48fedfad2d 100644
--- a/web/params.go
+++ b/web/params.go
@@ -5,6 +5,7 @@ package web
 
 import (
 	"net/http"
+	"net/url"
 	"strconv"
 	"strings"
 
@@ -255,14 +256,7 @@ func ParamsFromRequest(r *http.Request) *Params {
 		params.Permanent = val
 	}
 
-	if val, err := strconv.Atoi(query.Get("per_page")); err != nil || val < 0 {
-		params.PerPage = PerPageDefault
-	} else if val > PerPageMaximum {
-		params.PerPage = PerPageMaximum
-	} else {
-		params.PerPage = val
-	}
-
+	params.PerPage = getPerPageFromQuery(query)
 	if val, err := strconv.Atoi(query.Get("logs_per_page")); err != nil || val < 0 {
 		params.LogsPerPage = LogsPerPageDefault
 	} else if val > LogsPerPageMaximum {
@@ -363,3 +357,26 @@ func ParamsFromRequest(r *http.Request) *Params {
 
 	return params
 }
+
+// getPerPageFromQuery returns the PerPage value from the given query.
+// This function should be removed and the support for `pageSize`
+// should be dropped after v1.46 of the mobile app is no longer supported
+// https://mattermost.atlassian.net/browse/MM-38131
+func getPerPageFromQuery(query url.Values) int {
+	val, err := strconv.Atoi(query.Get("per_page"))
+	if err != nil {
+		val, err = strconv.Atoi(query.Get("pageSize"))
+		// if err != nil || val < 0 {
+		// 	return PerPageDefault
+		// } else if val > PerPageMaximum {
+		// 	return PerPageMaximum
+		// }
+		// return val
+	}
+	if err != nil || val < 0 {
+		return PerPageDefault
+	} else if val > PerPageMaximum {
+		return PerPageMaximum
+	}
+	return val
+}
diff --git a/web/params_test.go b/web/params_test.go
new file mode 100644
index 0000000000..54d8d82ddc
--- /dev/null
+++ b/web/params_test.go
@@ -0,0 +1,34 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+package web
+
+import (
+	"net/url"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetPerPageFromQuery(t *testing.T) {
+	t.Run("defaults should be set", func(t *testing.T) {
+		query := make(url.Values)
+		perPage := getPerPageFromQuery(query)
+		require.Equal(t, PerPageDefault, perPage)
+	})
+
+	t.Run("per_page should take priority", func(t *testing.T) {
+		query := make(url.Values)
+		query.Add("pageSize", "100")
+		query.Add("per_page", "50")
+		perPage := getPerPageFromQuery(query)
+		require.Equal(t, 50, perPage)
+	})
+
+	t.Run("pageSize should be used only if per_page is incorrectly set", func(t *testing.T) {
+		query := make(url.Values)
+		query.Add("pageSize", "100")
+		query.Add("per_page", "BAD VALUE")
+		perPage := getPerPageFromQuery(query)
+		require.Equal(t, 100, perPage)
+	})
+}