diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/InstantPrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/InstantPrototype.cpp
index 0fcee65d6e..b4d80ec349 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/InstantPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/InstantPrototype.cpp
@@ -42,6 +42,7 @@ void InstantPrototype::initialize(GlobalObject& global_object)
     define_native_function(vm.names.equals, equals, 1, attr);
     define_native_function(vm.names.toString, to_string, 0, attr);
     define_native_function(vm.names.toLocaleString, to_locale_string, 0, attr);
+    define_native_function(vm.names.toJSON, to_json, 0, attr);
     define_native_function(vm.names.valueOf, value_of, 0, attr);
 }
 
@@ -354,6 +355,23 @@ JS_DEFINE_NATIVE_FUNCTION(InstantPrototype::to_locale_string)
     return js_string(vm, *string);
 }
 
+// 8.3.15 Temporal.Instant.prototype.toJSON ( ), https://tc39.es/proposal-temporal/#sec-temporal.instant.prototype.tojson
+JS_DEFINE_NATIVE_FUNCTION(InstantPrototype::to_json)
+{
+    // 1. Let instant be the this value.
+    // 2. Perform ? RequireInternalSlot(instant, [[InitializedTemporalInstant]]).
+    auto* instant = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Return ? TemporalInstantToString(instant, undefined, "auto").
+    auto string = temporal_instant_to_string(global_object, *instant, js_undefined(), String { "auto"sv });
+    if (vm.exception())
+        return {};
+
+    return js_string(vm, *string);
+}
+
 // 8.3.16 Temporal.Instant.prototype.valueOf ( ), https://tc39.es/proposal-temporal/#sec-temporal.instant.prototype.valueof
 JS_DEFINE_NATIVE_FUNCTION(InstantPrototype::value_of)
 {
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/InstantPrototype.h b/Userland/Libraries/LibJS/Runtime/Temporal/InstantPrototype.h
index 9f769f1713..6149e6a7f6 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/InstantPrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/InstantPrototype.h
@@ -29,6 +29,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(equals);
     JS_DECLARE_NATIVE_FUNCTION(to_string);
     JS_DECLARE_NATIVE_FUNCTION(to_locale_string);
+    JS_DECLARE_NATIVE_FUNCTION(to_json);
     JS_DECLARE_NATIVE_FUNCTION(value_of);
 };
 
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.toJSON.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.toJSON.js
new file mode 100644
index 0000000000..63f965ac49
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.toJSON.js
@@ -0,0 +1,18 @@
+describe("correct behavior", () => {
+    test("length is 0", () => {
+        expect(Temporal.Instant.prototype.toJSON).toHaveLength(0);
+    });
+
+    test("basic functionality", () => {
+        const instant = new Temporal.Instant(1625614921123456789n);
+        expect(instant.toJSON()).toBe("2021-07-06T23:42:01.123456789Z");
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.Instant object", () => {
+        expect(() => {
+            Temporal.Instant.prototype.toJSON.call("foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.Instant");
+    });
+});