diff --git a/Userland/Libraries/LibJS/Runtime/BigInt.cpp b/Userland/Libraries/LibJS/Runtime/BigInt.cpp
index 8e78e3b81a..60022cccef 100644
--- a/Userland/Libraries/LibJS/Runtime/BigInt.cpp
+++ b/Userland/Libraries/LibJS/Runtime/BigInt.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2020-2021, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -7,6 +7,7 @@
 #include <LibCrypto/BigInt/SignedBigInteger.h>
 #include <LibJS/Heap/Heap.h>
 #include <LibJS/Runtime/BigInt.h>
+#include <LibJS/Runtime/GlobalObject.h>
 
 namespace JS {
 
@@ -25,4 +26,20 @@ BigInt* js_bigint(Heap& heap, Crypto::SignedBigInteger big_integer)
     return heap.allocate_without_global_object<BigInt>(move(big_integer));
 }
 
+// 21.2.1.1.1 NumberToBigInt ( number ), https://tc39.es/ecma262/#sec-numbertobigint
+BigInt* number_to_bigint(GlobalObject& global_object, Value number)
+{
+    VERIFY(number.is_number());
+    auto& vm = global_object.vm();
+
+    // 1. If IsIntegralNumber(number) is false, throw a RangeError exception.
+    if (!number.is_integral_number()) {
+        vm.throw_exception<RangeError>(global_object, ErrorType::BigIntIntArgument);
+        return {};
+    }
+
+    // 2. Return the BigInt value that represents ℝ(number).
+    return js_bigint(vm.heap(), Crypto::SignedBigInteger::create_from((i64)number.as_double()));
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Runtime/BigInt.h b/Userland/Libraries/LibJS/Runtime/BigInt.h
index fd1673fb5b..add9fae0c2 100644
--- a/Userland/Libraries/LibJS/Runtime/BigInt.h
+++ b/Userland/Libraries/LibJS/Runtime/BigInt.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2020-2021, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -26,5 +26,6 @@ private:
 };
 
 BigInt* js_bigint(Heap&, Crypto::SignedBigInteger);
+BigInt* number_to_bigint(GlobalObject&, Value);
 
 }
diff --git a/Userland/Libraries/LibJS/Runtime/BigIntConstructor.cpp b/Userland/Libraries/LibJS/Runtime/BigIntConstructor.cpp
index 19e644186c..dd83b83cf7 100644
--- a/Userland/Libraries/LibJS/Runtime/BigIntConstructor.cpp
+++ b/Userland/Libraries/LibJS/Runtime/BigIntConstructor.cpp
@@ -1,11 +1,11 @@
 /*
- * Copyright (c) 2020, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2020-2021, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #include <AK/String.h>
-#include <LibCrypto/BigInt/SignedBigInteger.h>
+#include <LibJS/Runtime/BigInt.h>
 #include <LibJS/Runtime/BigIntConstructor.h>
 #include <LibJS/Runtime/BigIntObject.h>
 #include <LibJS/Runtime/Error.h>
@@ -42,20 +42,22 @@ BigIntConstructor::~BigIntConstructor()
 // 21.2.1.1 BigInt ( value ), https://tc39.es/ecma262/#sec-bigint-constructor-number-value
 Value BigIntConstructor::call()
 {
-    auto primitive = vm().argument(0).to_primitive(global_object(), Value::PreferredType::Number);
-    if (vm().exception())
+    auto& vm = this->vm();
+    auto& global_object = this->global_object();
+
+    auto value = vm.argument(0);
+
+    // 2. Let prim be ? ToPrimitive(value, number).
+    auto primitive = value.to_primitive(global_object, Value::PreferredType::Number);
+    if (vm.exception())
         return {};
-    if (primitive.is_number()) {
-        if (!primitive.is_integral_number()) {
-            vm().throw_exception<RangeError>(global_object(), ErrorType::BigIntIntArgument);
-            return {};
-        }
-        return js_bigint(heap(), Crypto::SignedBigInteger { primitive.as_i32() });
-    }
-    auto* bigint = vm().argument(0).to_bigint(global_object());
-    if (vm().exception())
-        return {};
-    return bigint;
+
+    // 3. If Type(prim) is Number, return ? NumberToBigInt(prim).
+    if (primitive.is_number())
+        return number_to_bigint(global_object, primitive);
+
+    // 4. Otherwise, return ? ToBigInt(value).
+    return value.to_bigint(global_object);
 }
 
 // 21.2.1.1 BigInt ( value ), https://tc39.es/ecma262/#sec-bigint-constructor-number-value