diff --git a/.changeset/fresh-needles-join.md b/.changeset/fresh-needles-join.md
new file mode 100644
index 00000000000..fa8b3fce052
--- /dev/null
+++ b/.changeset/fresh-needles-join.md
@@ -0,0 +1,24 @@
+---
+"remix": patch
+"@remix-run/dev": patch
+---
+
+use `index` without leading underscore for colocation
+
+when using flat routes with folder colocation, use `index` without leading underscore for route
+
+before:
+```
+  app_.projects.$id.roadmap/
+    _index.tsx
+    chart.tsx
+    update-timeline.server.tsx
+```
+
+after:
+```
+  app_.projects.$id.roadmap/
+    index.tsx
+    chart.tsx
+    update-timeline.server.tsx
+```
diff --git a/packages/remix-dev/__tests__/flat-routes-test.ts b/packages/remix-dev/__tests__/flat-routes-test.ts
index 8735e752556..5446afec45a 100644
--- a/packages/remix-dev/__tests__/flat-routes-test.ts
+++ b/packages/remix-dev/__tests__/flat-routes-test.ts
@@ -4,6 +4,7 @@ import {
   createRoutePath,
   flatRoutesUniversal,
   getRouteSegments,
+  isIndexRoute,
 } from "../config/flat-routes";
 import type { ConfigRoute } from "../config/routes";
 
@@ -25,10 +26,10 @@ describe("flatRoutes", () => {
       ["nested/$slug", "/nested/:slug"],
       ["flat.$slug", "/flat/:slug"],
       ["flat.sub", "/flat/sub"],
-      ["nested/_index", "/nested"],
+      ["nested/index", "/nested"],
       ["flat._index", "/flat"],
       ["_index", undefined],
-      ["_layout/_index", undefined],
+      ["_layout/index", undefined],
       ["_layout/test", "/test"],
       ["_layout.test", "/test"],
       ["_layout/$slug", "/:slug"],
@@ -88,7 +89,8 @@ describe("flatRoutes", () => {
     for (let [input, expected] of tests) {
       it(`"${input}" -> "${expected}"`, () => {
         let routeSegments = getRouteSegments(input);
-        expect(createRoutePath(routeSegments)).toBe(expected);
+        let isIndex = isIndexRoute(input);
+        expect(createRoutePath(routeSegments, isIndex)).toBe(expected);
       });
     }
 
@@ -332,9 +334,9 @@ describe("flatRoutes", () => {
         },
       ],
       [
-        "routes/app_.skipall_/_index.tsx",
+        "routes/app_.skipall_/index.tsx",
         {
-          id: "routes/app_.skipall_/_index",
+          id: "routes/app_.skipall_/index",
           index: true,
           parentId: "root",
           path: "app/skipall",
@@ -580,9 +582,9 @@ describe("flatRoutes", () => {
         },
       ],
       [
-        "routes/brand/_index.tsx",
+        "routes/brand/index.tsx",
         {
-          id: "routes/brand/_index",
+          id: "routes/brand/index",
           parentId: "root",
           path: "brand",
           index: true,
diff --git a/packages/remix-dev/config/flat-routes.ts b/packages/remix-dev/config/flat-routes.ts
index 65d59e39406..16af3738278 100644
--- a/packages/remix-dev/config/flat-routes.ts
+++ b/packages/remix-dev/config/flat-routes.ts
@@ -103,8 +103,11 @@ export function flatRoutesUniversal(
   return routes;
 }
 
-function isIndexRoute(routeId: string) {
-  return routeId.endsWith("_index");
+export function isIndexRoute(routeId: string) {
+  let isFlatFile = !routeId.includes(path.posix.sep);
+  return isFlatFile
+    ? routeId.endsWith("_index")
+    : /\/index$/.test(routeId);
 }
 
 type State =
@@ -262,7 +265,7 @@ function getRouteInfo(
   let routeIdWithoutRoutes = routeId.slice(routeDirectory.length + 1);
   let index = isIndexRoute(routeIdWithoutRoutes);
   let routeSegments = getRouteSegments(routeIdWithoutRoutes);
-  let routePath = createRoutePath(routeSegments);
+  let routePath = createRoutePath(routeSegments, index);
 
   return {
     id: routeIdWithoutRoutes,
@@ -274,9 +277,13 @@ function getRouteInfo(
   };
 }
 
-export function createRoutePath(routeSegments: string[]) {
+export function createRoutePath(routeSegments: string[], isIndex: boolean) {
   let result = "";
 
+  if (isIndex) {
+    routeSegments = routeSegments.slice(0, -1);
+  }
+
   for (let segment of routeSegments) {
     // skip pathless layout segments
     if (segment.startsWith("_")) {