diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1419827ae..124aad641 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,6 +31,7 @@
 - Hovering over a function definition will now display the function signature,
   or the type of the hovered argument.
 - Use `import type` for importing types from typescript declarations.
+- Use `.d.mts` extension for typescript declarations to match `.mjs`.
 
 ## v0.30.4 - 2023-07-26
 
diff --git a/compiler-core/src/codegen.rs b/compiler-core/src/codegen.rs
index b35f1428e..f8d6dc932 100644
--- a/compiler-core/src/codegen.rs
+++ b/compiler-core/src/codegen.rs
@@ -181,7 +181,7 @@ impl<'a> JavaScript<'a> {
         tracing::debug!("Generated JS prelude");
         if self.typescript == TypeScriptDeclarations::Emit {
             writer.write(
-                &self.output_directory.join("gleam.d.ts"),
+                &self.output_directory.join("gleam.d.mts"),
                 javascript::PRELUDE_TS_DEF,
             )?;
             tracing::debug!("Generated TS prelude");
@@ -195,7 +195,7 @@ impl<'a> JavaScript<'a> {
         module: &Module,
         js_name: &str,
     ) -> Result<()> {
-        let name = format!("{js_name}.d.ts");
+        let name = format!("{js_name}.d.mts");
         let path = self.output_directory.join(name);
         let output = javascript::ts_declaration(&module.ast, &module.input_path, &module.code);
         tracing::debug!(name = ?js_name, "Generated TS declaration");
diff --git a/compiler-core/src/javascript.rs b/compiler-core/src/javascript.rs
index a0e9276f4..705aa4e97 100644
--- a/compiler-core/src/javascript.rs
+++ b/compiler-core/src/javascript.rs
@@ -21,8 +21,8 @@ use self::import::{Imports, Member};
 
 const INDENT: isize = 2;
 
-pub const PRELUDE: &str = include_str!("../templates/prelude.js");
-pub const PRELUDE_TS_DEF: &str = include_str!("../templates/prelude.d.ts");
+pub const PRELUDE: &str = include_str!("../templates/prelude.mjs");
+pub const PRELUDE_TS_DEF: &str = include_str!("../templates/prelude.d.mts");
 
 pub type Output<'a> = Result<Document<'a>, Error>;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bit_strings__bit_string_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bit_strings__bit_string_typescript.snap
index 675f3662a..a27c08c51 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bit_strings__bit_string_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bit_strings__bit_string_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/bit_strings.rs
 assertion_line: 148
 expression: "\npub fn go(x) {\n  <<x:bit_string, \"Gleam\":utf8>>\n}\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export function go(x: _.BitString): _.BitString;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bit_strings__utf8_codepoint_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bit_strings__utf8_codepoint_typescript.snap
index 1dbb9f3d1..ec74ddca9 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bit_strings__utf8_codepoint_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bit_strings__utf8_codepoint_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/bit_strings.rs
 assertion_line: 126
 expression: "\npub fn go(x) {\n  <<x:utf8_codepoint, \"Gleam\":utf8>>\n}\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export function go(x: _.UtfCodepoint): _.BitString;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bools__shadowed_bools_and_nil_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bools__shadowed_bools_and_nil_typescript.snap
index 45e8b78b4..054e80582 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bools__shadowed_bools_and_nil_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bools__shadowed_bools_and_nil_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/bools.rs
 assertion_line: 81
 expression: "\npub type True { True False Nil }\npub fn go(x, y) {\n  let assert True = x\n  let assert False = x\n  let assert Nil = y\n}\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export class True extends _.CustomType {}
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__const_with_fields_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__const_with_fields_typescript.snap
index 25653f67a..93256538e 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__const_with_fields_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__const_with_fields_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/custom_types.rs
 assertion_line: 154
 expression: "\npub type Mine {\n  Mine(a: Int, b: Int)\n}\n\npub const labels = Mine(b: 2, a: 1)\npub const no_labels = Mine(3, 4)\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export class Mine extends _.CustomType {
   constructor(a: number, b: number);
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__long_name_variant_mixed_labels_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__long_name_variant_mixed_labels_typescript.snap
index de8153af0..12f7817ae 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__long_name_variant_mixed_labels_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__long_name_variant_mixed_labels_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/custom_types.rs
 assertion_line: 225
 expression: "\npub type TypeWithALongNameAndSeveralArguments{\n  TypeWithALongNameAndSeveralArguments(String, String, String, a: String, b: String)\n}\n\npub const local = TypeWithALongNameAndSeveralArguments(\"one\", \"two\", \"three\", \"four\", \"five\")\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export class TypeWithALongNameAndSeveralArguments extends _.CustomType {
   constructor(
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__opaque_types_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__opaque_types_typescript.snap
index 02635b1ab..4b3fe72ef 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__opaque_types_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__opaque_types_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/custom_types.rs
 assertion_line: 662
 expression: "pub opaque type Animal {\n  Cat(goes_outside: Bool)\n  Dog(plays_fetch: Bool)\n}\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 class Cat extends _.CustomType {
   constructor(goes_outside: boolean);
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unapplied_record_constructors_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unapplied_record_constructors_typescript.snap
index 8add7ed1a..e4ac36c28 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unapplied_record_constructors_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unapplied_record_constructors_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/custom_types.rs
 assertion_line: 650
 expression: "pub type Cat { Cat(name: String) }\n\npub fn return_unapplied_cat() {\n  Cat\n}\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export class Cat extends _.CustomType {
   constructor(name: string);
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unnamed_fields_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unnamed_fields_typescript.snap
index 5fb7a4b46..e7f087bb8 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unnamed_fields_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unnamed_fields_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/custom_types.rs
 assertion_line: 195
 expression: "\npub type Ip{\n    Ip(String)\n}\n\npub const local = Ip(\"0.0.0.0\")\n\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export class Ip extends _.CustomType {
   constructor(argument$0: string);
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unqualified_imported_no_label_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unqualified_imported_no_label_typescript.snap
index ed169e706..9d4c17c3a 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unqualified_imported_no_label_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unqualified_imported_no_label_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/custom_types.rs
 assertion_line: 416
 expression: "import other.{Two}\npub fn main() {\n  Two(1)\n}"
 ---
-import type * as other from "../other.d.ts";
+import type * as other from "../other.d.mts";
 
 export function main(): other.One$;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_typscript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_typscript.snap
index bc8326366..51787b9bc 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_typscript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_typscript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/custom_types.rs
 assertion_line: 49
 expression: "import other\npub fn main() {\n  other.Two\n}"
 ---
-import type * as other from "../other.d.ts";
+import type * as other from "../other.d.mts";
 
 export function main(): other.One$;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_unqualified_aliased_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_unqualified_aliased_typescript.snap
index 7f4ef58e0..834446fb2 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_unqualified_aliased_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_unqualified_aliased_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/custom_types.rs
 assertion_line: 93
 expression: "import other.{Two as Three}\npub fn main() {\n  Three\n}"
 ---
-import type * as other from "../other.d.ts";
+import type * as other from "../other.d.mts";
 
 export function main(): other.One$;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_unqualified_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_unqualified_typescript.snap
index 74df94638..5020d7be5 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_unqualified_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__zero_arity_imported_unqualified_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/custom_types.rs
 assertion_line: 71
 expression: "import other.{Two}\npub fn main() {\n  Two\n}"
 ---
-import type * as other from "../other.d.ts";
+import type * as other from "../other.d.mts";
 
 export function main(): other.One$;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__generics__record_generics_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__generics__record_generics_typescript.snap
index 7b64a8f7e..a11d6b62b 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__generics__record_generics_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__generics__record_generics_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/generics.rs
 assertion_line: 15
 expression: "pub type Animal(t) {\n  Cat(type_: t)\n  Dog(type_: t)\n}\n\npub fn main() {\n  Cat(type_: 6)\n}\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export class Cat<I> extends _.CustomType {
   constructor(type_: I);
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__generics__result_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__generics__result_typescript.snap
index 0b3f85e21..7c3dfb3a4 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__generics__result_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__generics__result_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/generics.rs
 assertion_line: 40
 expression: "pub fn map(result, fun) {\n            case result {\n              Ok(a) -> Ok(fun(a))\n              Error(e) -> Error(e)\n            }\n          }"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export function map<S, T, V>(result: _.Result<T, S>, fun: (x0: T) => V): _.Result<
   V,
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__lists__list_constants_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__lists__list_constants_typescript.snap
index 18df4b4e8..d7a28041f 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__lists__list_constants_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__lists__list_constants_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/lists.rs
 assertion_line: 52
 expression: "\npub const a = []\npub const b = [1, 2, 3]\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export const a: _.List<any>;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__modules__imported_custom_types_do_get_rendered_in_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__modules__imported_custom_types_do_get_rendered_in_typescript.snap
index f6f903a8e..e6908deba 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__modules__imported_custom_types_do_get_rendered_in_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__modules__imported_custom_types_do_get_rendered_in_typescript.snap
@@ -3,8 +3,8 @@ source: compiler-core/src/javascript/tests/modules.rs
 assertion_line: 182
 expression: "import one/two/three.{Custom, One, Two}\n\npub fn go() -> List(Custom) { [One, Two] }\n"
 ---
-import type * as _ from "../gleam.d.ts";
-import type * as three from "../one/two/three.d.ts";
+import type * as _ from "../gleam.d.mts";
+import type * as three from "../one/two/three.d.mts";
 
 export function go(): _.List<three.Custom$>;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__prelude__qualified_nil_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__prelude__qualified_nil_typescript.snap
index 0b00de7f7..70f272460 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__prelude__qualified_nil_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__prelude__qualified_nil_typescript.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/prelude.rs
 assertion_line: 41
 expression: "import gleam\npub fn go() { gleam.Nil }\n"
 ---
-import type * as gleam from "../gleam.d.ts";
+import type * as gleam from "../gleam.d.mts";
 
 export function go(): null;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__prelude__qualified_ok_typescript.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__prelude__qualified_ok_typescript.snap
index 7a208f891..405b2851c 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__prelude__qualified_ok_typescript.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__prelude__qualified_ok_typescript.snap
@@ -3,8 +3,8 @@ source: compiler-core/src/javascript/tests/prelude.rs
 assertion_line: 14
 expression: "import gleam\npub fn go() { gleam.Ok(1) }\n"
 ---
-import type * as _ from "../gleam.d.ts";
-import type * as gleam from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
+import type * as gleam from "../gleam.d.mts";
 
 export function go(): _.Result<number, any>;
 
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__type_alias__type_alias.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__type_alias__type_alias.snap
index 17a96b1bb..a1e331003 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__type_alias__type_alias.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__type_alias__type_alias.snap
@@ -3,7 +3,7 @@ source: compiler-core/src/javascript/tests/type_alias.rs
 assertion_line: 5
 expression: "\npub type Headers = List(#(String, String))\n"
 ---
-import type * as _ from "../gleam.d.ts";
+import type * as _ from "../gleam.d.mts";
 
 export type Headers = _.List<[string, string]>;
 
diff --git a/compiler-core/src/javascript/typescript.rs b/compiler-core/src/javascript/typescript.rs
index 18d8ec424..0c4323ac5 100644
--- a/compiler-core/src/javascript/typescript.rs
+++ b/compiler-core/src/javascript/typescript.rs
@@ -280,16 +280,16 @@ impl<'a> TypeScriptGenerator<'a> {
         if package == self.module.type_info.package || package.is_empty() {
             // Same package
             match self.current_module_name_segments_count {
-                1 => format!("./{module}.d.ts"),
+                1 => format!("./{module}.d.mts"),
                 _ => {
                     let prefix = "../".repeat(self.current_module_name_segments_count - 1);
-                    format!("{prefix}{module}.d.ts")
+                    format!("{prefix}{module}.d.mts")
                 }
             }
         } else {
             // Different package
             let prefix = "../".repeat(self.current_module_name_segments_count);
-            format!("{prefix}{package}/{module}.d.ts")
+            format!("{prefix}{package}/{module}.d.mts")
         }
     }
 
diff --git a/compiler-core/templates/prelude.d.ts b/compiler-core/templates/prelude.d.mts
similarity index 100%
rename from compiler-core/templates/prelude.d.ts
rename to compiler-core/templates/prelude.d.mts
diff --git a/compiler-core/templates/prelude.js b/compiler-core/templates/prelude.mjs
similarity index 100%
rename from compiler-core/templates/prelude.js
rename to compiler-core/templates/prelude.mjs
diff --git a/test-package-compiler/src/lib.rs b/test-package-compiler/src/lib.rs
index f5dc30c1c..db5a25592 100644
--- a/test-package-compiler/src/lib.rs
+++ b/test-package-compiler/src/lib.rs
@@ -110,7 +110,7 @@ impl TestCompileOutput {
             match content {
                 _ if extension == Some("cache") => buffer.push_str("<.cache binary>"),
 
-                _ if path.ends_with("gleam.mjs") || path.ends_with("gleam.d.ts") => {
+                _ if path.ends_with("gleam.mjs") || path.ends_with("gleam.d.mts") => {
                     buffer.push_str("<prelude>")
                 }
 
diff --git a/test-package-compiler/src/snapshots/test_package_compiler__generated_tests__javascript_d_ts.snap b/test-package-compiler/src/snapshots/test_package_compiler__generated_tests__javascript_d_ts.snap
index f3b66b3f8..4ecfa69af 100644
--- a/test-package-compiler/src/snapshots/test_package_compiler__generated_tests__javascript_d_ts.snap
+++ b/test-package-compiler/src/snapshots/test_package_compiler__generated_tests__javascript_d_ts.snap
@@ -9,14 +9,14 @@ expression: "./cases/javascript_d_ts"
 //// /out/lib/the_package/_gleam_artefacts/hello.cache_meta
 <29 byte binary>
 
-//// /out/lib/the_package/gleam.d.ts
+//// /out/lib/the_package/gleam.d.mts
 <prelude>
 
 //// /out/lib/the_package/gleam.mjs
 <prelude>
 
-//// /out/lib/the_package/hello.d.ts
-import type * as _ from "./gleam.d.ts";
+//// /out/lib/the_package/hello.d.mts
+import type * as _ from "./gleam.d.mts";
 
 export class Woo extends _.CustomType {}
 
diff --git a/test-package-compiler/src/snapshots/test_package_compiler__generated_tests__javascript_import.snap b/test-package-compiler/src/snapshots/test_package_compiler__generated_tests__javascript_import.snap
index 397b57cd5..de7dafe3e 100644
--- a/test-package-compiler/src/snapshots/test_package_compiler__generated_tests__javascript_import.snap
+++ b/test-package-compiler/src/snapshots/test_package_compiler__generated_tests__javascript_import.snap
@@ -15,14 +15,14 @@ expression: "./cases/javascript_import"
 //// /out/lib/the_package/_gleam_artefacts/two.cache_meta
 <44 byte binary>
 
-//// /out/lib/the_package/gleam.d.ts
+//// /out/lib/the_package/gleam.d.mts
 <prelude>
 
 //// /out/lib/the_package/gleam.mjs
 <prelude>
 
-//// /out/lib/the_package/one/two.d.ts
-import type * as _ from "../gleam.d.ts";
+//// /out/lib/the_package/one/two.d.mts
+import type * as _ from "../gleam.d.mts";
 
 export class A extends _.CustomType {}
 
@@ -35,8 +35,8 @@ import { CustomType as $CustomType } from "../gleam.mjs";
 export class A extends $CustomType {}
 
 
-//// /out/lib/the_package/two.d.ts
-import type * as two from "./one/two.d.ts";
+//// /out/lib/the_package/two.d.mts
+import type * as two from "./one/two.d.mts";
 
 export const x: two.A$;
 
diff --git a/test/javascript_prelude/Makefile b/test/javascript_prelude/Makefile
index 09bdebb04..88cd7b20f 100644
--- a/test/javascript_prelude/Makefile
+++ b/test/javascript_prelude/Makefile
@@ -1,6 +1,6 @@
 .PHONY: test
 test:
 	@echo test/javascript_prelude
-	@cp ../../compiler-core/templates/prelude.js prelude.mjs
+	@cp ../../compiler-core/templates/prelude.mjs prelude.mjs
 	@node main.mjs
 	@rm prelude.mjs