diff --git a/Userland/Libraries/LibJS/Runtime/DatePrototype.cpp b/Userland/Libraries/LibJS/Runtime/DatePrototype.cpp
index 7d11f24e90e..e75979609d6 100644
--- a/Userland/Libraries/LibJS/Runtime/DatePrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/DatePrototype.cpp
@@ -81,6 +81,7 @@ void DatePrototype::initialize(GlobalObject& global_object)
     define_native_function(vm.names.toLocaleTimeString, to_locale_time_string, 0, attr);
     define_native_function(vm.names.toTimeString, to_time_string, 0, attr);
     define_native_function(vm.names.toString, to_string, 0, attr);
+    define_native_function(vm.names.toJSON, to_json, 1, attr);
 
     define_native_function(vm.well_known_symbol_to_primitive(), symbol_to_primitive, 1, Attribute::Configurable);
 
@@ -91,11 +92,6 @@ void DatePrototype::initialize(GlobalObject& global_object)
     // The function object that is the initial value of Date.prototype.toGMTString
     // is the same function object that is the initial value of Date.prototype.toUTCString.
     define_property(vm.names.toGMTString, get(vm.names.toUTCString), attr);
-
-    // toJSON() isn't quite an alias for toISOString():
-    // - it returns null instead of throwing RangeError
-    // - its .length is 1, not 0
-    // - it can be transferred to other prototypes
 }
 
 DatePrototype::~DatePrototype()
@@ -764,6 +760,19 @@ JS_DEFINE_NATIVE_FUNCTION(DatePrototype::to_string)
     return js_string(vm, move(string));
 }
 
+JS_DEFINE_NATIVE_FUNCTION(DatePrototype::to_json)
+{
+    auto* this_object = vm.this_value(global_object).to_object(global_object);
+    if (!this_object)
+        return {};
+
+    auto time_value = Value(this_object).to_primitive(global_object, Value::PreferredType::Number);
+    if (time_value.is_number() && !time_value.is_finite_number())
+        return js_null();
+
+    return this_object->invoke(vm.names.toISOString);
+}
+
 JS_DEFINE_NATIVE_FUNCTION(DatePrototype::symbol_to_primitive)
 {
     auto this_value = vm.this_value(global_object);
diff --git a/Userland/Libraries/LibJS/Runtime/DatePrototype.h b/Userland/Libraries/LibJS/Runtime/DatePrototype.h
index 9d9ec317e5a..5256659fb91 100644
--- a/Userland/Libraries/LibJS/Runtime/DatePrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/DatePrototype.h
@@ -56,6 +56,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(to_locale_time_string);
     JS_DECLARE_NATIVE_FUNCTION(to_time_string);
     JS_DECLARE_NATIVE_FUNCTION(to_string);
+    JS_DECLARE_NATIVE_FUNCTION(to_json);
     JS_DECLARE_NATIVE_FUNCTION(symbol_to_primitive);
 };