diff --git a/src/Call/OneClick/InitOneClickPaymentRequest.php b/src/Call/OneClick/InitOneClickPaymentRequest.php
index b0beb47..75de925 100644
--- a/src/Call/OneClick/InitOneClickPaymentRequest.php
+++ b/src/Call/OneClick/InitOneClickPaymentRequest.php
@@ -10,6 +10,7 @@
 use SlevomatCsobGateway\Crypto\SignatureDataFormatter;
 use SlevomatCsobGateway\Price;
 use SlevomatCsobGateway\Validator;
+use function base64_encode;
 
 class InitOneClickPaymentRequest
 {
@@ -32,13 +33,17 @@ class InitOneClickPaymentRequest
 	/** @var string */
 	private $clientIp;
 
+	/** @var string|null */
+	private $merchantData;
+
 	public function __construct(
 		string $merchantId,
 		string $origPayId,
 		string $orderId,
 		string $clientIp,
 		?Price $price = null,
-		?string $description = null
+		?string $description = null,
+		?string $merchantData = null
 	)
 	{
 		Validator::checkPayId($origPayId);
@@ -46,6 +51,9 @@ public function __construct(
 		if ($description !== null) {
 			Validator::checkDescription($description);
 		}
+		if ($merchantData !== null) {
+			Validator::checkMerchantData($merchantData);
+		}
 
 		$this->merchantId = $merchantId;
 		$this->origPayId = $origPayId;
@@ -53,6 +61,7 @@ public function __construct(
 		$this->clientIp = $clientIp;
 		$this->price = $price;
 		$this->description = $description;
+		$this->merchantData = $merchantData;
 	}
 
 	public function send(ApiClient $apiClient): PaymentResponse
@@ -73,6 +82,10 @@ public function send(ApiClient $apiClient): PaymentResponse
 			$requestData['description'] = $this->description;
 		}
 
+		if ($this->merchantData !== null) {
+			$requestData['merchantData'] = base64_encode($this->merchantData);
+		}
+
 		$response = $apiClient->post(
 			'oneclick/init',
 			$requestData,
@@ -85,6 +98,7 @@ public function send(ApiClient $apiClient): PaymentResponse
 				'totalAmount' => null,
 				'currency' => null,
 				'description' => null,
+				'merchantData' => null,
 			]),
 			new SignatureDataFormatter([
 				'payId' => null,
diff --git a/src/RequestFactory.php b/src/RequestFactory.php
index 7abe117..efd763a 100644
--- a/src/RequestFactory.php
+++ b/src/RequestFactory.php
@@ -150,7 +150,8 @@ public function createOneclickInitPayment(
 		string $orderId,
 		string $clientIp,
 		?Price $price = null,
-		?string $description = null
+		?string $description = null,
+		?string $merchantData = null
 	): InitOneClickPaymentRequest
 	{
 		return new InitOneClickPaymentRequest(
@@ -159,7 +160,8 @@ public function createOneclickInitPayment(
 			$orderId,
 			$clientIp,
 			$price,
-			$description
+			$description,
+			$merchantData
 		);
 	}
 
diff --git a/tests/unit/Call/OneClick/InitOneClickPaymentRequestTest.php b/tests/unit/Call/OneClick/InitOneClickPaymentRequestTest.php
index bcc6d27..1652900 100644
--- a/tests/unit/Call/OneClick/InitOneClickPaymentRequestTest.php
+++ b/tests/unit/Call/OneClick/InitOneClickPaymentRequestTest.php
@@ -12,6 +12,7 @@
 use SlevomatCsobGateway\Call\ResultCode;
 use SlevomatCsobGateway\Currency;
 use SlevomatCsobGateway\Price;
+use function base64_encode;
 
 class InitOneClickPaymentRequestTest extends TestCase
 {
@@ -32,6 +33,7 @@ public function testSend(): void
 				'totalAmount' => 1789600,
 				'currency' => 'CZK',
 				'description' => 'Nákup na vasobchod.cz (Lenovo ThinkPad Edge E540, Doprava PPL)',
+				'merchantData' => base64_encode('some-base64-encoded-merchant-data'),
 			])
 			->willReturn(
 				new Response(ResponseCode::get(ResponseCode::S200_OK), [
@@ -49,7 +51,8 @@ public function testSend(): void
 			'5547',
 			'127.0.0.1',
 			new Price(1789600, Currency::get(Currency::CZK)),
-			'Nákup na vasobchod.cz (Lenovo ThinkPad Edge E540, Doprava PPL)'
+			'Nákup na vasobchod.cz (Lenovo ThinkPad Edge E540, Doprava PPL)',
+			'some-base64-encoded-merchant-data'
 		);
 
 		$paymentResponse = $initPaymentRequest->send($apiClient);
diff --git a/tests/unit/RequestFactoryTest.php b/tests/unit/RequestFactoryTest.php
index 0f8c20a..590ac46 100644
--- a/tests/unit/RequestFactoryTest.php
+++ b/tests/unit/RequestFactoryTest.php
@@ -116,7 +116,8 @@ public function testCreateOneclickInitPayment(): void
 			'5547123',
 			'127.0.0.1',
 			new Price(1789600, Currency::get(Currency::CZK)),
-			'Nákup na vasobchod.cz (Lenovo ThinkPad Edge E540, Doprava PPL)'
+			'Nákup na vasobchod.cz (Lenovo ThinkPad Edge E540, Doprava PPL)',
+			'some-base64-encoded-merchant-data'
 		);
 
 		self::assertTrue(true);