diff --git a/include/utils.h b/include/utils.h
index f9be579..d432507 100644
--- a/include/utils.h
+++ b/include/utils.h
@@ -19,6 +19,7 @@ uint16_t double_to_half(double value);
 
 char hex_char(int v);
 char *resolve_js_file(const char *filename);
+char *resolve_typescript_source_fallback(const char *filename);
 
 int hex_digit(char c);
 int is_typescript_file(const char *filename);
diff --git a/src/esm/loader.c b/src/esm/loader.c
index b25fd42..f1308e5 100644
--- a/src/esm/loader.c
+++ b/src/esm/loader.c
@@ -162,6 +162,21 @@ static char *esm_try_resolve_index_with_exts(const char *dir, const char *spec)
   return esm_try_resolve_with_exts(dir, idx, false);
 }
 
+static char *esm_try_resolve_relative_typescript_source_fallback(
+  const char *dir,
+  const char *spec,
+  const char *base_path
+) {
+  if (!is_typescript_file(base_path)) return NULL;
+
+  char *ts_spec = resolve_typescript_source_fallback(spec);
+  if (!ts_spec) return NULL;
+
+  char *resolved = esm_try_resolve(dir, ts_spec, "");
+  free(ts_spec);
+  return resolved;
+}
+
 static ant_value_t esm_default_export_or_namespace(ant_t *js, ant_value_t ns) {
   ant_value_t default_val = js_get_slot(ns, SLOT_DEFAULT);
   return vtype(default_val) != T_UNDEF ? default_val : ns;
@@ -631,10 +646,14 @@ static char *esm_resolve_relative_path(const char *specifier, const char *base_p
 
   const char *spec = specifier;
   if (strncmp(specifier, "./", 2) == 0) spec = specifier + 2;
+  
   bool has_ext = esm_has_extension(spec);
-
   if ((result = esm_try_resolve(dir, spec, ""))) goto cleanup;
-  if (has_ext) goto cleanup;
+  
+  if (has_ext) {
+    result = esm_try_resolve_relative_typescript_source_fallback(dir, spec, base_path);
+    goto cleanup;
+  }
 
   char *base_ext = esm_get_extension(base_path);
   if (!base_ext) goto cleanup;
diff --git a/src/utils.c b/src/utils.c
index c6ee8a2..12b8113 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -252,6 +252,32 @@ char *resolve_js_file(const char *filename) {
   return NULL;
 }
 
+char *resolve_typescript_source_fallback(const char *filename) {
+  if (!filename) return NULL;
+
+  const char *mapped_ext = NULL;
+  size_t trim_len = 0;
+
+  size_t len = strlen(filename);
+  if (len > 3 && strcmp(filename + len - 3, ".js") == 0) {
+    mapped_ext = ".ts";
+    trim_len = 3;
+  } else if (len > 4 && strcmp(filename + len - 4, ".mjs") == 0) {
+    mapped_ext = ".mts";
+    trim_len = 4;
+  } else if (len > 4 && strcmp(filename + len - 4, ".cjs") == 0) {
+    mapped_ext = ".cts";
+    trim_len = 4;
+  } else return NULL;
+
+  size_t mapped_len = strlen(mapped_ext);
+  char *mapped = try_oom(len - trim_len + mapped_len + 1);
+  memcpy(mapped, filename, len - trim_len);
+  memcpy(mapped + len - trim_len, mapped_ext, mapped_len + 1);
+  
+  return mapped;
+}
+
 typedef struct {
   const char *repl; size_t repl_len; size_t *ri;
   const char *matched; size_t matched_len;
diff --git a/tests/test_ts_js_extension_resolution.cjs b/tests/test_ts_js_extension_resolution.cjs
new file mode 100644
index 0000000..16e85bf
--- /dev/null
+++ b/tests/test_ts_js_extension_resolution.cjs
@@ -0,0 +1,10 @@
+(async () => {
+  const mod = await import('./ts_js_extension_resolution_main.ts');
+  if (mod.default !== 'ts-source-fallback-ok') {
+    throw new Error(`expected ts-source-fallback-ok, got ${String(mod.default)}`);
+  }
+  console.log('ok');
+})().catch(err => {
+  console.error(err);
+  process.exit(1);
+});
diff --git a/tests/ts_js_extension_resolution_dep.ts b/tests/ts_js_extension_resolution_dep.ts
new file mode 100644
index 0000000..28d0fdf
--- /dev/null
+++ b/tests/ts_js_extension_resolution_dep.ts
@@ -0,0 +1 @@
+export const resolvedValue = 'ts-source-fallback-ok';
diff --git a/tests/ts_js_extension_resolution_main.ts b/tests/ts_js_extension_resolution_main.ts
new file mode 100644
index 0000000..43ecd6b
--- /dev/null
+++ b/tests/ts_js_extension_resolution_main.ts
@@ -0,0 +1,3 @@
+import { resolvedValue } from './ts_js_extension_resolution_dep.js';
+
+export default resolvedValue;