diff --git a/py/builtinimport.c b/py/builtinimport.c
index d8127a66f0..f6c2f7c348 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -428,6 +428,20 @@ STATIC mp_obj_t process_import_at_level(qstr full_mod_name, qstr level_mod_name,
     } else {
         DEBUG_printf("Searching for sub-module\n");
 
+        #if MICROPY_MODULE_BUILTIN_SUBPACKAGES
+        // If the outer module is a built-in (because its map is in ROM), then
+        // treat it like a package if it contains this submodule in its
+        // globals dict.
+        mp_obj_module_t *mod = MP_OBJ_TO_PTR(outer_module_obj);
+        if (mod->globals->map.is_fixed) {
+            elem = mp_map_lookup(&mod->globals->map, MP_OBJ_NEW_QSTR(level_mod_name), MP_MAP_LOOKUP);
+            // Also verify that the entry in the globals dict is in fact a module.
+            if (elem && mp_obj_is_type(elem->value, &mp_type_module)) {
+                return elem->value;
+            }
+        }
+        #endif
+
         // If the outer module is a package, it will have __path__ set.
         // We can use that as the path to search inside.
         mp_obj_t dest[2];
diff --git a/py/mpconfig.h b/py/mpconfig.h
index f3d5ce557a..292fafb52e 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -850,6 +850,20 @@ typedef double mp_float_t;
 #define MICROPY_MODULE_BUILTIN_INIT (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EXTRA_FEATURES)
 #endif
 
+// Whether to allow built-in modules to have sub-packages (by making the
+// sub-package a member of their locals dict). Sub-packages should not be
+// registered with MP_REGISTER_MODULE, instead they should be added as
+// members of the parent's globals dict. To match CPython behavior,
+// their __name__ should be "foo.bar"(i.e. QSTR_foo_dot_bar) which will
+// require an entry in qstrdefs, although it does also work to just call
+// it "bar". Also, because subpackages can be accessed without being
+// imported (e.g. as foo.bar after `import foo`), they should not
+// have __init__ methods. Instead, the top-level package's __init__ should
+// initialise all sub-packages.
+#ifndef MICROPY_MODULE_BUILTIN_SUBPACKAGES
+#define MICROPY_MODULE_BUILTIN_SUBPACKAGES (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EVERYTHING)
+#endif
+
 // Whether to support module-level __getattr__ (see PEP 562)
 #ifndef MICROPY_MODULE_GETATTR
 #define MICROPY_MODULE_GETATTR (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_CORE_FEATURES)