diff --git a/jscomp/bsb/bsb_config_load.ml b/jscomp/bsb/bsb_config_load.ml
index e947c920ed1..02556637631 100644
--- a/jscomp/bsb/bsb_config_load.ml
+++ b/jscomp/bsb/bsb_config_load.ml
@@ -17,8 +17,7 @@ let load_json ~(per_proj_dir : string) ~(warn_legacy_config : bool)
       | exception _ -> raise e (* forward error from rescript.json *)
   in
   if warn_legacy_config && filename = Literals.bsconfig_json then
-    (* TODO: warn legacy config *)
-    ();
+    print_endline "Warning: bsconfig.json is deprecated. Migrate it to rescript.json\n";
   match Ext_json_parse.parse_json_from_chan abs in_chan
   with
   | v -> close_in in_chan ; (filename, v)
diff --git a/jscomp/bsb/bsb_config_parse.ml b/jscomp/bsb/bsb_config_parse.ml
index 1a6f654ba70..747a2b7e085 100644
--- a/jscomp/bsb/bsb_config_parse.ml
+++ b/jscomp/bsb/bsb_config_parse.ml
@@ -237,7 +237,11 @@ let extract_js_post_build (map : json_map) cwd : string option =
 
 (** ATT: make sure such function is re-entrant.
     With a given [cwd] it works anywhere*)
-let interpret_json ~(package_kind : Bsb_package_kind.t) ~(per_proj_dir : string) ~(warn_legacy_config : bool)
+let interpret_json
+  ~(filename : string)
+  ~(json : Ext_json_types.t)
+  ~(package_kind : Bsb_package_kind.t)
+  ~(per_proj_dir : string)
     : Bsb_config_types.t =
   (* we should not resolve it too early,
       since it is external configuration, no {!Bsb_build_util.convert_and_resolve_path}
@@ -253,10 +257,9 @@ let interpret_json ~(package_kind : Bsb_package_kind.t) ~(per_proj_dir : string)
      1. if [build.ninja] does use [ninja] we need set a variable
      2. we need store it so that we can call ninja correctly
   *)
-  match
-    Bsb_config_load.load_json ~per_proj_dir ~warn_legacy_config
+  match json
   with
-  | filename, Obj { map } -> (
+  | Obj { map } -> (
       let package_name, namespace = extract_package_name_and_namespace map in
       let gentype_config = extract_gentype_config map in
 
@@ -353,7 +356,7 @@ let interpret_json ~(package_kind : Bsb_package_kind.t) ~(per_proj_dir : string)
           }
       | None ->
           Bsb_exception.invalid_spec ("no sources specified in " ^ filename))
-  | filename, _ -> Bsb_exception.invalid_spec (filename ^ " expect a json object {}")
+  | _ -> Bsb_exception.invalid_spec (filename ^ " expect a json object {}")
 
 let deps_from_bsconfig () =
   let cwd = Bsb_global_paths.cwd in
diff --git a/jscomp/bsb/bsb_config_parse.mli b/jscomp/bsb/bsb_config_parse.mli
index 84e18eb1ff0..3aedf9891b1 100644
--- a/jscomp/bsb/bsb_config_parse.mli
+++ b/jscomp/bsb/bsb_config_parse.mli
@@ -25,4 +25,8 @@
 val deps_from_bsconfig : unit -> Bsb_package_specs.t * Bsb_jsx.t * bool *  Set_string.t
 
 val interpret_json :
-  package_kind:Bsb_package_kind.t -> per_proj_dir:string -> warn_legacy_config:bool -> Bsb_config_types.t
+  filename:string ->
+  json:Ext_json_types.t ->
+  package_kind:Bsb_package_kind.t ->
+  per_proj_dir:string ->
+  Bsb_config_types.t
diff --git a/jscomp/bsb/bsb_ninja_regen.ml b/jscomp/bsb/bsb_ninja_regen.ml
index 009918d8c14..030e98b953f 100644
--- a/jscomp/bsb/bsb_ninja_regen.ml
+++ b/jscomp/bsb/bsb_ninja_regen.ml
@@ -38,6 +38,9 @@ let regenerate_ninja ~(package_kind : Bsb_package_kind.t) ~forced ~per_proj_dir
   let check_result =
     Bsb_ninja_check.check ~package_kind ~per_proj_dir ~forced ~file:output_deps
   in
+  let config_filename, config_json =
+    Bsb_config_load.load_json ~per_proj_dir ~warn_legacy_config
+  in
   match check_result with
   | Good -> None (* Fast path, no need regenerate ninja *)
   | Bsb_forced | Bsb_bsc_version_mismatch | Bsb_package_kind_inconsistent
@@ -52,7 +55,8 @@ let regenerate_ninja ~(package_kind : Bsb_package_kind.t) ~forced ~per_proj_dir
         Bsb_clean.clean_self per_proj_dir);
 
       let config : Bsb_config_types.t =
-        Bsb_config_parse.interpret_json ~package_kind ~per_proj_dir ~warn_legacy_config
+        Bsb_config_parse.interpret_json
+          ~filename:config_filename ~json:config_json ~package_kind ~per_proj_dir
       in
       (* create directory, lib/bs, lib/js, lib/es6 etc *)
       Bsb_build_util.mkp lib_bs_dir;
diff --git a/jscomp/build_tests/warn_legacy_config/bsconfig.json b/jscomp/build_tests/warn_legacy_config/bsconfig.json
new file mode 100644
index 00000000000..ff8aa6b4b0f
--- /dev/null
+++ b/jscomp/build_tests/warn_legacy_config/bsconfig.json
@@ -0,0 +1,8 @@
+{
+  "name": "warn_legacy_config",
+  "version": "0.1.0",
+  "sources": {
+    "dir": "src",
+    "subdirs": true
+  }
+}
diff --git a/jscomp/build_tests/warn_legacy_config/input.js b/jscomp/build_tests/warn_legacy_config/input.js
new file mode 100644
index 00000000000..ad81e9ec498
--- /dev/null
+++ b/jscomp/build_tests/warn_legacy_config/input.js
@@ -0,0 +1,10 @@
+const { spawnSync } = require("child_process");
+const assert = require("assert");
+const rescript_exe = require("../../../scripts/bin_path").rescript_exe;
+
+const output = spawnSync(rescript_exe, { encoding: "utf8" });
+assert(
+  /^Warning: bsconfig.json is deprecated. Migrate it to rescript.json/.test(
+    output.stdout
+  )
+);
diff --git a/jscomp/build_tests/warn_legacy_config/src/demo.res b/jscomp/build_tests/warn_legacy_config/src/demo.res
new file mode 100644
index 00000000000..8d0b19151fc
--- /dev/null
+++ b/jscomp/build_tests/warn_legacy_config/src/demo.res
@@ -0,0 +1 @@
+let () = Js.log("Hello, ReScript")