diff --git a/doc/changes/10869.md b/doc/changes/10869.md
new file mode 100644
index 000000000000..3467dd2ee337
--- /dev/null
+++ b/doc/changes/10869.md
@@ -0,0 +1,2 @@
+- Fix staged pps preprocessors on Windows (which were not working at all
+  previously) (#10869, @nojb)
diff --git a/otherlibs/stdune/src/string.ml b/otherlibs/stdune/src/string.ml
index 81438928f873..1420d5e26425 100644
--- a/otherlibs/stdune/src/string.ml
+++ b/otherlibs/stdune/src/string.ml
@@ -278,7 +278,22 @@ let need_quoting s =
 ;;
 
 let quote_for_shell s = if need_quoting s then Stdlib.Filename.quote s else s
-let quote_list_for_shell l = List.map l ~f:quote_for_shell |> concat ~sep:" "
+
+let quote_list_for_shell = function
+  | [] -> ""
+  | prog :: args ->
+    let prog =
+      if Sys.win32 && contains prog '/'
+      then
+        map
+          ~f:(function
+            | '/' -> '\\'
+            | c -> c)
+          prog
+      else prog
+    in
+    quote_for_shell prog :: List.map ~f:quote_for_shell args |> concat ~sep:" "
+;;
 
 let of_list chars =
   let s = Bytes.make (List.length chars) '0' in
diff --git a/otherlibs/stdune/src/string.mli b/otherlibs/stdune/src/string.mli
index 5caa864fadcf..8619861d2a69 100644
--- a/otherlibs/stdune/src/string.mli
+++ b/otherlibs/stdune/src/string.mli
@@ -116,8 +116,8 @@ val need_quoting : string -> bool
     [true] *)
 val quote_for_shell : string -> string
 
-(** [quote_list_for_shell l] is
-    [List.map l ~f:quote_for_shell |> concat ~sep:" "] *)
+(** [quote_list_for_shell l] quotes a command-line so that it can be passed to
+    the system shell (eg by using [Sys.command]). *)
 val quote_list_for_shell : string list -> string
 
 val filter_map : string -> f:(char -> char option) -> string
diff --git a/test/blackbox-tests/test-cases/dune b/test/blackbox-tests/test-cases/dune
index 630343865a75..59e394349d58 100644
--- a/test/blackbox-tests/test-cases/dune
+++ b/test/blackbox-tests/test-cases/dune
@@ -29,7 +29,7 @@
   (alias runtest-js)))
 
 (cram
- (applies_to windows-diff)
+ (applies_to windows-diff github6644)
  (alias runtest-windows))
 
 ; DISABLED TESTS