diff --git a/app/assets/javascripts/discourse/app/controllers/user-private-messages.js b/app/assets/javascripts/discourse/app/controllers/user-private-messages.js
index 45c37747bd2..3b66fa6b613 100644
--- a/app/assets/javascripts/discourse/app/controllers/user-private-messages.js
+++ b/app/assets/javascripts/discourse/app/controllers/user-private-messages.js
@@ -59,7 +59,10 @@ export default class extends Controller {
   get messagesDropdownContent() {
     const content = [
       {
-        id: this.router.urlFor("userPrivateMessages.user", this.model.username),
+        id: this.router.urlFor(
+          "userPrivateMessages.user",
+          this.model.username_lower
+        ),
         name: I18n.t("user.messages.inbox"),
       },
     ];
@@ -78,7 +81,10 @@ export default class extends Controller {
 
     if (this.pmTaggingEnabled) {
       content.push({
-        id: this.router.urlFor("userPrivateMessages.tags", this.model.username),
+        id: this.router.urlFor(
+          "userPrivateMessages.tags",
+          this.model.username_lower
+        ),
         name: I18n.t("user.messages.tags"),
         icon: "tags",
       });
@@ -86,7 +92,7 @@ export default class extends Controller {
 
     customUserNavMessagesDropdownRows.forEach((row) => {
       content.push({
-        id: this.router.urlFor(row.routeName, this.model.username),
+        id: this.router.urlFor(row.routeName, this.model.username_lower),
         name: row.name,
         icon: row.icon,
       });
diff --git a/app/assets/javascripts/discourse/tests/acceptance/user-private-messages-test.js b/app/assets/javascripts/discourse/tests/acceptance/user-private-messages-test.js
index 6875db5cf38..a320b7c2d31 100644
--- a/app/assets/javascripts/discourse/tests/acceptance/user-private-messages-test.js
+++ b/app/assets/javascripts/discourse/tests/acceptance/user-private-messages-test.js
@@ -19,6 +19,7 @@ import {
 } from "discourse/lib/topic-list-tracker";
 import { withPluginApi } from "discourse/lib/plugin-api";
 import { resetCustomUserNavMessagesDropdownRows } from "discourse/controllers/user-private-messages";
+import userFixtures from "discourse/tests/fixtures/user-fixtures";
 
 acceptance(
   "User Private Messages - user with no group messages",
@@ -1046,3 +1047,28 @@ acceptance(
     });
   }
 );
+
+acceptance(
+  "User Private Messages - user with uppercase username",
+  function (needs) {
+    needs.user({
+      redesigned_user_page_nav_enabled: true,
+    });
+
+    needs.pretender((server, helper) => {
+      const response = cloneJSON(userFixtures["/u/charlie.json"]);
+      response.user.username = "chArLIe";
+      server.get("/u/charlie.json", () => helper.response(response));
+    });
+
+    test("viewing inbox", async function (assert) {
+      await visit("/u/charlie/messages");
+
+      assert.strictEqual(
+        query(".user-nav-messages-dropdown .selected-name").textContent.trim(),
+        "Inbox",
+        "menu defaults to Inbox"
+      );
+    });
+  }
+);