diff --git a/apps/readest-app/src/__tests__/services/backup-windows-paths.test.ts b/apps/readest-app/src/__tests__/services/backup-windows-paths.test.ts
new file mode 100644
index 00000000..3482f754
--- /dev/null
+++ b/apps/readest-app/src/__tests__/services/backup-windows-paths.test.ts
@@ -0,0 +1,74 @@
+import { describe, it, expect } from 'vitest';
+import type { ZipWriter } from '@zip.js/zip.js';
+import { addBackupEntriesToZip } from '@/services/backupService';
+import type { AppService, FileItem } from '@/types/system';
+
+/**
+ * Regression test for issue #4703: a backup zip exported on Windows failed to
+ * restore on every platform. `readDirectory` returns paths using the host
+ * separator, so on Windows `file.path` is `hash\cover.png` (backslash). When
+ * that backslash leaked into the zip entry name, restore's
+ * `filename.startsWith(`${hash}/`)` filter (forward slash) never matched and
+ * every book file was silently skipped. Entry names must always use `/`.
+ */
+
+const BOOK_HASH = '6afdd0136531fbe028e0503a14ba234c';
+
+/** Build a stub AppService whose `readDirectory` returns the given file list. */
+function makeAppService(files: FileItem[]): AppService {
+  return {
+    loadLibraryBooks: async () => [],
+    loadSettings: async () => ({}) as never,
+    resolveFilePath: async () => 'C:/Users/me/AppData/Books',
+    readDirectory: async () => files,
+    readFile: async () => new ArrayBuffer(8),
+  } as unknown as AppService;
+}
+
+/** A ZipWriter stub that records the entry name passed to each `add` call. */
+function makeCapturingWriter() {
+  const names: string[] = [];
+  const writer = {
+    add: async (name: string) => {
+      names.push(name);
+    },
+  } as unknown as ZipWriter<unknown>;
+  return { writer, names };
+}
+
+describe('addBackupEntriesToZip - cross-platform entry names (#4703)', () => {
+  it('normalizes Windows backslash paths to forward slashes', async () => {
+    const windowsFiles: FileItem[] = [
+      { path: `${BOOK_HASH}\\book.epub`, size: 1000 },
+      { path: `${BOOK_HASH}\\cover.png`, size: 200 },
+      { path: `${BOOK_HASH}\\config.json`, size: 50 },
+    ];
+    const { writer, names } = makeCapturingWriter();
+
+    await addBackupEntriesToZip(writer, makeAppService(windowsFiles), {});
+
+    // No entry name may contain a backslash.
+    expect(names.some((n) => n.includes('\\'))).toBe(false);
+
+    // Restore filters book files by `${hash}/`; the Windows export must match.
+    const bookEntries = names.filter((n) => n.startsWith(`${BOOK_HASH}/`));
+    expect(bookEntries).toEqual([
+      `${BOOK_HASH}/book.epub`,
+      `${BOOK_HASH}/cover.png`,
+      `${BOOK_HASH}/config.json`,
+    ]);
+  });
+
+  it('leaves POSIX forward-slash paths unchanged', async () => {
+    const posixFiles: FileItem[] = [
+      { path: `${BOOK_HASH}/book.epub`, size: 1000 },
+      { path: `${BOOK_HASH}/cover.png`, size: 200 },
+    ];
+    const { writer, names } = makeCapturingWriter();
+
+    await addBackupEntriesToZip(writer, makeAppService(posixFiles), {});
+
+    const bookEntries = names.filter((n) => n.startsWith(`${BOOK_HASH}/`));
+    expect(bookEntries).toEqual([`${BOOK_HASH}/book.epub`, `${BOOK_HASH}/cover.png`]);
+  });
+});
diff --git a/apps/readest-app/src/services/backupService.ts b/apps/readest-app/src/services/backupService.ts
index 36d1c105..5c220100 100644
--- a/apps/readest-app/src/services/backupService.ts
+++ b/apps/readest-app/src/services/backupService.ts
@@ -250,7 +250,7 @@ type ProgressCallback = (current: number, total: number, filename: string) => vo
 /**
  * Shared logic: add all library entries to a ZipWriter.
  */
-async function addBackupEntriesToZip(
+export async function addBackupEntriesToZip(
   writer: ZipWriter<unknown>,
   appService: AppService,
   options: BackupOptions,
@@ -288,7 +288,12 @@ async function addBackupEntriesToZip(
     try {
       const content = await appService.readFile(file.path, 'Books', 'binary');
       const data = new Uint8Array(content as ArrayBuffer);
-      await writer.add(file.path, new Uint8ArrayReader(data), { level: 0 });
+      // `readDirectory` returns host-separator paths; on Windows that is a
+      // backslash (e.g. `hash\cover.png`). Zip entry names must use forward
+      // slashes so the backup restores on every platform — restore matches a
+      // book's files by `${hash}/` (see `restoreFromBackupZip`). Issue #4703.
+      const entryName = file.path.replace(/\\/g, '/');
+      await writer.add(entryName, new Uint8ArrayReader(data), { level: 0 });
     } catch (error) {
       console.warn(`Skipping file ${file.path}:`, error);
     }