diff --git a/pkgs/racket-doc/scribblings/foreign/misc.scrbl b/pkgs/racket-doc/scribblings/foreign/misc.scrbl
index 566b19f07f..03670206ef 100644
--- a/pkgs/racket-doc/scribblings/foreign/misc.scrbl
+++ b/pkgs/racket-doc/scribblings/foreign/misc.scrbl
@@ -26,11 +26,16 @@ internal representation of @racket[vec].}
 Returns a pointer to an array of @racket[_double] values, which is the
 internal representation of @racket[flvec].}
 
-@defproc[(saved-errno) exact-integer?]{
+@defproc*[([(saved-errno) exact-integer?]
+           [(saved-errno [new-value exact-integer?]) void?])]{
 
-Returns the value most recently saved (in the current thread) after a
-foreign call with a non-@racket[#f] @racket[#:save-errno] option (see
-@racket[_fun] and @racket[_cprocedure]).}
+Returns or sets the error code saved for the current Racket
+thread. The saved error code is set after a foreign call with a
+non-@racket[#f] @racket[#:save-errno] option (see @racket[_fun] and
+@racket[_cprocedure]), but it can also be set explicitly (for example,
+to create mock foreign functions for testing).
+
+@history[#:changed "6.4.0.9"]{Added the one-argument variant.}}
 
 @defproc[(lookup-errno [sym (or/c 'EINTR 'EEXIST 'EAGAIN)])
          exact-integer?]{
diff --git a/racket/src/foreign/foreign.c b/racket/src/foreign/foreign.c
index 01147ed9ff..805b571bdd 100644
--- a/racket/src/foreign/foreign.c
+++ b/racket/src/foreign/foreign.c
@@ -4290,7 +4290,16 @@ static void save_errno_values(int kind)
 static Scheme_Object *foreign_saved_errno(int argc, Scheme_Object *argv[])
 {
   Scheme_Thread *p = scheme_current_thread;
-  return scheme_make_integer_value(p->saved_errno);
+  if (argc == 0) {
+    return scheme_make_integer_value(p->saved_errno);
+  } else {
+    intptr_t v;
+    if (!scheme_get_int_val(argv[0], &v)) {
+      wrong_intptr(MYNAME, 0, argc, argv);
+    }
+    p->saved_errno = v;
+    return scheme_void;
+  }
 }
 #undef MYNAME
 
@@ -4552,7 +4561,7 @@ void scheme_init_foreign(Scheme_Env *env)
   scheme_add_global_constant("ffi-callback",
     scheme_make_noncm_prim(foreign_ffi_callback, "ffi-callback", 3, 6), menv);
   scheme_add_global_constant("saved-errno",
-    scheme_make_immed_prim(foreign_saved_errno, "saved-errno", 0, 0), menv);
+    scheme_make_immed_prim(foreign_saved_errno, "saved-errno", 0, 1), menv);
   scheme_add_global_constant("lookup-errno",
     scheme_make_immed_prim(foreign_lookup_errno, "lookup-errno", 1, 1), menv);
   scheme_add_global_constant("make-stubborn-will-executor",
@@ -4923,7 +4932,7 @@ void scheme_init_foreign(Scheme_Env *env)
   scheme_add_global_constant("ffi-callback",
    scheme_make_noncm_prim((Scheme_Prim *)unimplemented, "ffi-callback", 3, 6), menv);
   scheme_add_global_constant("saved-errno",
-   scheme_make_immed_prim((Scheme_Prim *)unimplemented, "saved-errno", 0, 0), menv);
+   scheme_make_immed_prim((Scheme_Prim *)unimplemented, "saved-errno", 0, 1), menv);
   scheme_add_global_constant("lookup-errno",
    scheme_make_immed_prim((Scheme_Prim *)unimplemented, "lookup-errno", 1, 1), menv);
   scheme_add_global_constant("make-stubborn-will-executor",
diff --git a/racket/src/foreign/foreign.rktc b/racket/src/foreign/foreign.rktc
index c9d81905c4..4581da12e6 100755
--- a/racket/src/foreign/foreign.rktc
+++ b/racket/src/foreign/foreign.rktc
@@ -3441,9 +3441,18 @@ static void save_errno_values(int kind)
   p->saved_errno = errno;
 }
 
-@cdefine[saved-errno 0 #:kind immed]{
+@cdefine[saved-errno 0 1 #:kind immed]{
   Scheme_Thread *p = scheme_current_thread;
-  return scheme_make_integer_value(p->saved_errno);
+  if (argc == 0) {
+    return scheme_make_integer_value(p->saved_errno);
+  } else {
+    intptr_t v;
+    if (!scheme_get_int_val(argv[0], &v)) {
+      wrong_intptr(MYNAME, 0, argc, argv);
+    }
+    p->saved_errno = v;
+    return scheme_void;
+  }
 }
 
 @cdefine[lookup-errno 1 #:kind immed]{