diff --git a/src/Init/System/IO.lean b/src/Init/System/IO.lean
index f84413cdf7..dfcbbad058 100644
--- a/src/Init/System/IO.lean
+++ b/src/Init/System/IO.lean
@@ -381,6 +381,23 @@ def appDir : m FilePath := do
 
 def currentDir : m FilePath := liftM Prim.currentDir
 
+@[extern "lean_io_create_dir"]
+constant createDir : @& FilePath → IO Unit
+
+partial def createDirAll (p : FilePath) : IO Unit := do
+  if ← p.isDir then
+    return ()
+  if let some parent := p.parent then
+    createDirAll parent
+  try
+    createDir p
+  catch
+    | e =>
+      if ← p.isDir then
+        pure ()  -- I guess someone else was faster
+      else
+        throw e
+
 end
 
 namespace Process
diff --git a/src/runtime/io.cpp b/src/runtime/io.cpp
index ee30c239f0..a33b723a82 100644
--- a/src/runtime/io.cpp
+++ b/src/runtime/io.cpp
@@ -536,6 +536,18 @@ extern "C" obj_res lean_io_metadata(b_obj_arg fname, obj_arg) {
     return io_result_mk_ok(mdata);
 }
 
+extern "C" obj_res lean_io_create_dir(b_obj_arg p, obj_arg) {
+#ifdef LEAN_WINDOWS
+    if (mkdir(string_cstr(p)) == 0) {
+#else
+    if (mkdir(string_cstr(p), 0777) == 0) {
+#endif
+        return io_result_mk_ok(box(0));
+    } else {
+        return io_result_mk_error(decode_io_error(errno, p));
+    }
+}
+
 extern "C" obj_res lean_io_remove_file(b_obj_arg fname, obj_arg) {
     if (std::remove(string_cstr(fname)) == 0) {
         return io_result_mk_ok(box(0));