diff --git a/crates/engine/src/autosave.rs b/crates/engine/src/autosave.rs
index 5b87a958..45d462f0 100644
--- a/crates/engine/src/autosave.rs
+++ b/crates/engine/src/autosave.rs
@@ -470,7 +470,17 @@ impl Preferences {
     }
 }
 
-/// The per-user data directory (`FILMCRAFT_DATA_DIR` overrides):
+/// A file with this name next to the executable makes the install portable: its data directory is
+/// `<exe dir>/data` instead of the per-user one (the Windows portable zip ships it).
+pub const PORTABLE_MARKER: &str = "portable.txt";
+
+/// `<exe_dir>/data` when `exe_dir` holds [`PORTABLE_MARKER`], else `None`.
+pub fn portable_data_dir(exe_dir: &Path) -> Option<PathBuf> {
+    exe_dir.join(PORTABLE_MARKER).is_file().then(|| exe_dir.join("data"))
+}
+
+/// The data directory. `FILMCRAFT_DATA_DIR` overrides; a portable install ([`PORTABLE_MARKER`] next
+/// to the executable) uses `<exe dir>/data`; otherwise the per-user directory:
 /// macOS `~/Library/Application Support/FilmCraft`, Windows `%APPDATA%\FilmCraft`,
 /// elsewhere `$XDG_DATA_HOME/filmcraft` or `~/.local/share/filmcraft`.
 pub fn default_data_dir() -> Option<PathBuf> {
@@ -478,6 +488,9 @@ pub fn default_data_dir() -> Option<PathBuf> {
     if let Some(d) = env("FILMCRAFT_DATA_DIR") {
         return Some(d);
     }
+    if let Some(d) = std::env::current_exe().ok().and_then(|exe| exe.parent().and_then(portable_data_dir)) {
+        return Some(d);
+    }
     if cfg!(target_os = "macos") {
         return env("HOME").map(|h| h.join("Library/Application Support/FilmCraft"));
     }
diff --git a/crates/engine/src/autosave_tests.rs b/crates/engine/src/autosave_tests.rs
index 0e113e41..5ac8281e 100644
--- a/crates/engine/src/autosave_tests.rs
+++ b/crates/engine/src/autosave_tests.rs
@@ -176,6 +176,19 @@ fn auto_save_ring_rotates_and_can_save_the_project() {
     let _ = std::fs::remove_dir_all(&d);
 }
 
+#[test]
+fn portable_marker_keeps_data_next_to_the_executable() {
+    // #139: a portable install must not write to %APPDATA%.
+    let d = temp_dir("portable");
+    assert_eq!(autosave::portable_data_dir(&d), None);
+    std::fs::create_dir_all(d.join(autosave::PORTABLE_MARKER)).unwrap();
+    assert_eq!(autosave::portable_data_dir(&d), None, "a directory is not the marker");
+    std::fs::remove_dir_all(d.join(autosave::PORTABLE_MARKER)).unwrap();
+    std::fs::write(d.join(autosave::PORTABLE_MARKER), "").unwrap();
+    assert_eq!(autosave::portable_data_dir(&d), Some(d.join("data")));
+    let _ = std::fs::remove_dir_all(&d);
+}
+
 #[test]
 fn journal_costs_the_ui_thread_only_a_handoff() {
     // Per change the UI thread pays an Arc clone and a channel send; encoding happens on the worker.
diff --git a/docs/project-files.md b/docs/project-files.md
index 68980cea..b7db090a 100644
--- a/docs/project-files.md
+++ b/docs/project-files.md
@@ -159,7 +159,8 @@ Desktop flags: `--recover` recovers the newest candidate without asking. `--no-r
 without asking; the changes stay available through File ▸ Recover Unsaved Changes…. `--data-dir
 <dir>` (or `FILMCRAFT_DATA_DIR`) moves the data directory. The default is `~/Library/Application
 Support/FilmCraft` on macOS, `%APPDATA%\FilmCraft` on Windows, and `$XDG_DATA_HOME/filmcraft` or
-`~/.local/share/filmcraft` elsewhere.
+`~/.local/share/filmcraft` elsewhere. A `portable.txt` file next to the executable (the Windows
+portable zip ships one) makes it `data` next to the executable instead.
 
 ### Cost
 
diff --git a/packaging/windows/package.ps1 b/packaging/windows/package.ps1
index 04b31efb..faa4b863 100644
--- a/packaging/windows/package.ps1
+++ b/packaging/windows/package.ps1
@@ -5,7 +5,7 @@
 .DESCRIPTION
   Produces, in $env:DIST (default: dist/release):
     filmcraft-<version>-windows-<arch>.msi            per-machine installer (WiX v5)
-    filmcraft-<version>-windows-<arch>-portable.zip   filmcraft.exe + filmcraft-cli.exe
+    filmcraft-<version>-windows-<arch>-portable.zip   filmcraft.exe + filmcraft-cli.exe + portable.txt
 
   The binaries link the C runtime statically (+crt-static), so neither the MSI nor the portable
   zip needs the Visual C++ redistributable. Signing is delegated to sign.ps1 (skipped with a
@@ -106,6 +106,11 @@ $Portable = Join-Path $TargetDir "windows-package\filmcraft-$Version-windows-$Ar
 Remove-Item -Recurse -Force $Portable -ErrorAction SilentlyContinue
 New-Item -ItemType Directory -Force -Path $Portable | Out-Null
 Copy-Item (Join-Path $Stage '*.exe') $Portable
+# The marker that makes FilmCraft keep its data in .\data next to the exe instead of %APPDATA% (#139).
+Set-Content -Encoding ascii -Path (Join-Path $Portable 'portable.txt') -Value @(
+  'FilmCraft portable: preferences, presets, logs and crash recovery are kept in the data folder',
+  'next to filmcraft.exe. Delete this file to use %APPDATA%\FilmCraft instead.'
+)
 foreach ($f in 'README.md', 'LICENSE', 'LICENSE-MIT', 'LICENSE-APACHE') {
   $p = Join-Path $Root $f
   if (Test-Path $p) { Copy-Item $p $Portable }