diff --git a/.changeset/rsbuild-windows-route-manifest.md b/.changeset/rsbuild-windows-route-manifest.md
new file mode 100644
index 0000000000..ec78789008
--- /dev/null
+++ b/.changeset/rsbuild-windows-route-manifest.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/start-plugin-core': patch
+---
+
+Fix the Rsbuild Start manifest dropping every route's stylesheets and preloads on Windows by normalizing rspack module paths to the POSIX form the generated route tree uses.
diff --git a/packages/start-plugin-core/src/rsbuild/normalized-client-build.ts b/packages/start-plugin-core/src/rsbuild/normalized-client-build.ts
index 60244fdca9..9660cdaa8e 100644
--- a/packages/start-plugin-core/src/rsbuild/normalized-client-build.ts
+++ b/packages/start-plugin-core/src/rsbuild/normalized-client-build.ts
@@ -12,6 +12,15 @@ type RspackCompilation = Rspack.Compilation
 type RspackCompilationChunk = Rspack.Chunk
 type RspackModule = Rspack.Module
 
+const backslashRegex = /\\/g
+
+/**
+ * Convert an OS native path to the POSIX form used by the generated route tree.
+ */
+function toPosixPath(filePath: string): string {
+  return filePath.replace(backslashRegex, '/')
+}
+
 /**
  * Extract route file paths from rspack module identifiers.
  *
@@ -42,7 +51,14 @@ function getRouteFilePathsFromModules(
     if (!new URLSearchParams(query).has(tsrSplit)) continue
 
     const nameForCondition = mod.nameForCondition()
-    const routeFilePath = nameForCondition ?? resourcePart.slice(0, queryIndex)
+
+    // rspack reports module paths using the OS separator, while the generated route
+    // tree records every route `filePath` with POSIX separators. Normalize before the
+    // path becomes a manifest key, otherwise no route matches its chunk on Windows and
+    // every route loses its stylesheets and preloads.
+    const routeFilePath = toPosixPath(
+      nameForCondition ?? resourcePart.slice(0, queryIndex),
+    )
 
     if (seen?.has(routeFilePath)) continue
 
diff --git a/packages/start-plugin-core/tests/rsbuild/normalized-client-build.test.ts b/packages/start-plugin-core/tests/rsbuild/normalized-client-build.test.ts
new file mode 100644
index 0000000000..08ae2fd923
--- /dev/null
+++ b/packages/start-plugin-core/tests/rsbuild/normalized-client-build.test.ts
@@ -0,0 +1,106 @@
+import { describe, expect, test } from 'vitest'
+import { normalizeRspackClientBuild } from '../../src/rsbuild/normalized-client-build'
+import { buildStartManifest } from '../../src/start-manifest-plugin/manifestBuilder'
+import type { Rspack } from '@rsbuild/core'
+
+function makeModule(options: {
+  identifier: string
+  nameForCondition?: string | null
+}): Rspack.Module {
+  return {
+    identifier: () => options.identifier,
+    nameForCondition: () => options.nameForCondition ?? null,
+  } as unknown as Rspack.Module
+}
+
+function makeChunk(options: {
+  name: string
+  files: Array<string>
+  auxiliaryFiles?: Array<string>
+}): Rspack.Chunk {
+  const chunk = {
+    name: options.name,
+    files: new Set(options.files),
+    auxiliaryFiles: new Set(options.auxiliaryFiles ?? []),
+  } as unknown as Rspack.Chunk
+
+  const group = {
+    chunks: [chunk],
+    childrenIterable: new Set(),
+  }
+
+  ;(chunk as any).groupsIterable = new Set([group])
+
+  return chunk
+}
+
+function makeCompilation(
+  entries: Array<{ chunk: Rspack.Chunk; modules: Array<Rspack.Module> }>,
+): Rspack.Compilation {
+  const entryChunk = entries.find((entry) => entry.chunk.name === 'index')!
+  const modulesByChunk = new Map(
+    entries.map((entry) => [entry.chunk, entry.modules]),
+  )
+
+  return {
+    entrypoints: new Map([['index', { chunks: [entryChunk.chunk] }]]),
+    chunks: new Set(entries.map((entry) => entry.chunk)),
+    chunkGraph: {
+      getChunkModules: (chunk: Rspack.Chunk) => modulesByChunk.get(chunk) ?? [],
+    },
+    getAssets: () => [],
+  } as unknown as Rspack.Compilation
+}
+
+function makeWindowsCompilation() {
+  return makeCompilation([
+    {
+      chunk: makeChunk({ name: 'index', files: ['index.js'] }),
+      modules: [makeModule({ identifier: 'C:\\app\\src\\client.tsx' })],
+    },
+    {
+      chunk: makeChunk({
+        name: 'posts',
+        files: ['posts.js'],
+        auxiliaryFiles: ['posts.css'],
+      }),
+      modules: [
+        makeModule({
+          identifier:
+            'builtin:swc-loader??ruleSet[0]!C:\\app\\src\\routes\\posts.tsx?tsr-split=component',
+          nameForCondition: 'C:\\app\\src\\routes\\posts.tsx',
+        }),
+      ],
+    },
+  ])
+}
+
+describe('normalizeRspackClientBuild', () => {
+  test('keys route chunks by a POSIX path when rspack reports OS native paths', () => {
+    const build = normalizeRspackClientBuild(makeWindowsCompilation())
+
+    expect([...build.chunkFileNamesByRouteFilePath.keys()]).toEqual([
+      'C:/app/src/routes/posts.tsx',
+    ])
+    expect(build.chunksByFileName.get('posts.js')?.routeFilePaths).toEqual([
+      'C:/app/src/routes/posts.tsx',
+    ])
+  })
+
+  test('gives a route its stylesheet when rspack reports OS native paths', () => {
+    const manifest = buildStartManifest({
+      clientBuild: normalizeRspackClientBuild(makeWindowsCompilation()),
+      routeTreeRoutes: {
+        __root__: {
+          filePath: 'C:/app/src/routes/__root.tsx',
+          children: ['/posts'],
+        },
+        '/posts': { filePath: 'C:/app/src/routes/posts.tsx', children: [] },
+      },
+      basePath: '/',
+    })
+
+    expect(manifest.routes['/posts']?.css).toEqual(['/posts.css'])
+    expect(manifest.routes['/posts']?.preloads).toEqual(['/posts.js'])
+  })
+})