diff --git a/.changeset/slimy-lobsters-live.md b/.changeset/slimy-lobsters-live.md
new file mode 100644
index 0000000..0dc0dfd
--- /dev/null
+++ b/.changeset/slimy-lobsters-live.md
@@ -0,0 +1,5 @@
+---
+'@mysten/bcs': minor
+---
+
+Allow BcsType.transform to omit input or output transform
diff --git a/packages/bcs/src/bcs-type.ts b/packages/bcs/src/bcs-type.ts
index 5ab84d3..2c92f53 100644
--- a/packages/bcs/src/bcs-type.ts
+++ b/packages/bcs/src/bcs-type.ts
@@ -79,24 +79,25 @@ export class BcsType<T, Input = T> {
 		return this.parse(fromBase64(b64));
 	}
 
-	transform<T2, Input2>({
+	transform<T2 = T, Input2 = Input>({
 		name,
 		input,
 		output,
 		validate,
 	}: {
-		input: (val: Input2) => Input;
-		output: (value: T) => T2;
+		input?: (val: Input2) => Input;
+		output?: (value: T) => T2;
 	} & BcsTypeOptions<T2, Input2>) {
 		return new BcsType<T2, Input2>({
 			name: name ?? this.name,
-			read: (reader) => output(this.read(reader)),
-			write: (value, writer) => this.#write(input(value), writer),
-			serializedSize: (value) => this.serializedSize(input(value)),
-			serialize: (value, options) => this.#serialize(input(value), options),
+			read: (reader) => (output ? output(this.read(reader)) : (this.read(reader) as never)),
+			write: (value, writer) => this.#write(input ? input(value) : (value as never), writer),
+			serializedSize: (value) => this.serializedSize(input ? input(value) : (value as never)),
+			serialize: (value, options) =>
+				this.#serialize(input ? input(value) : (value as never), options),
 			validate: (value) => {
 				validate?.(value);
-				this.validate(input(value));
+				this.validate(input ? input(value) : (value as never));
 			},
 		});
 	}
diff --git a/packages/bcs/tests/builder.test.ts b/packages/bcs/tests/builder.test.ts
index b91962e..0240ee4 100644
--- a/packages/bcs/tests/builder.test.ts
+++ b/packages/bcs/tests/builder.test.ts
@@ -249,6 +249,31 @@ describe('bcs', () => {
 			Variant2: 'hello',
 		});
 	});
+
+	describe('transform', () => {
+		const stringU8 = bcs.u8().transform({
+			input: (val: string) => parseInt(val),
+			output: (val) => val.toString(),
+		});
+
+		testType('transform', stringU8, '1', '01', '1');
+
+		// Output only
+		const bigIntu64 = bcs.u64().transform({
+			output: (val) => BigInt(val),
+		});
+
+		testType('transform', bigIntu64, '1', '0100000000000000', 1n);
+		testType('transform', bigIntu64, 1, '0100000000000000', 1n);
+		testType('transform', bigIntu64, 1n, '0100000000000000', 1n);
+
+		// Input only
+		const hexU8 = bcs.u8().transform({
+			input: (val: string) => Number.parseInt(val, 16),
+		});
+
+		testType('transform', hexU8, 'ff', 'ff', 255);
+	});
 });
 
 function testType<T, Input>(