diff --git a/pack.js b/pack.js
index 9fc4a66..dca72c7 100644
--- a/pack.js
+++ b/pack.js
@@ -532,8 +532,26 @@ export class Packr extends Unpackr {
 					if (this.largeBigIntToFloat) {
 						target[position++] = 0xcb
 						targetView.setFloat64(position, Number(value))
+					} else if (this.useBigIntExtension && value < 2n**(1023n) && value > -(2n**(1023n))) {
+						target[position++] = 0xc7
+						position++;
+						target[position++] = 0x42 // "B" for BigInt
+						let bytes = [];
+						let alignedSign;
+						do {
+							let byte = value & 0xffn;
+							alignedSign = (byte & 0x80n) === (value < 0n ? 0x80n : 0n);
+							bytes.push(byte);
+							value >>= 8n;
+						} while (!((value === 0n || value === -1n) && alignedSign));
+						target[position-2] = bytes.length;
+						for (let i = bytes.length; i > 0;) {
+							target[position++] = Number(bytes[--i]);
+						}
+						return
 					} else {
-						throw new RangeError(value + ' was too large to fit in MessagePack 64-bit integer format, set largeBigIntToFloat to convert to float-64')
+						throw new RangeError(value + ' was too large to fit in MessagePack 64-bit integer format, use' +
+							' useBigIntExtension or set largeBigIntToFloat to convert to float-64')
 					}
 				}
 				position += 8
diff --git a/tests/test.js b/tests/test.js
index ef193c8..f9c278f 100644
--- a/tests/test.js
+++ b/tests/test.js
@@ -374,6 +374,22 @@ suite('msgpackr basic tests', function() {
 		assert.equal(pack(123).length, 1)
 	})
 
+	test('BigInt', function() {
+		let packr = new Packr({useBigIntExtension: true})
+		let data = {
+			a: 3333333333333333333333333333n,
+			b: 1234567890123456789012345678901234567890n,
+			c: -3333333333333333333333333333n,
+			d: -352523523642364364364264264264264264262642642n,
+			e: 0xffffffffffffffffffffffffffn,
+			f: -0xffffffffffffffffffffffffffn,
+		}
+		let serialized = packr.pack(data)
+		let deserialized = packr.unpack(serialized)
+		assert.deepEqual(data, deserialized)
+	})
+
+
 	test('extended class pack/unpack', function(){
 		function Extended() {
 
diff --git a/unpack.js b/unpack.js
index 3e9e616..b019a02 100644
--- a/unpack.js
+++ b/unpack.js
@@ -988,6 +988,17 @@ const recordDefinition = (id, highByte) => {
 currentExtensions[0] = () => {} // notepack defines extension 0 to mean undefined, so use that as the default here
 currentExtensions[0].noBuffer = true
 
+currentExtensions[0x42] = (data) => {
+	// decode bigint
+	let length = data.length;
+	let value = BigInt(data[0] & 0x80 ? data[0] - 0x100 : data[0]);
+	for (let i = 1; i < length; i++) {
+		value <<= 8n;
+		value += BigInt(data[i]);
+	}
+	return value;
+}
+
 let errors = { Error, TypeError, ReferenceError };
 currentExtensions[0x65] = () => {
 	let data = read()