diff --git a/src/ConvertKit_API.php b/src/ConvertKit_API.php
index 84d1cc1..8eb4e92 100644
--- a/src/ConvertKit_API.php
+++ b/src/ConvertKit_API.php
@@ -10,6 +10,7 @@
 use Monolog\Logger;
 use Monolog\Handler\StreamHandler;
 use GuzzleHttp\Client;
+use GuzzleHttp\ClientInterface;
 use GuzzleHttp\Psr7\Request;
 
 /**
@@ -67,9 +68,9 @@ class ConvertKit_API
     protected $debug_logger;
 
     /**
-     * Guzzle Http Client
+     * Guzzle Http ClientInterface
      *
-     * @var \GuzzleHttp\Client
+     * @var \GuzzleHttp\ClientInterface
      */
     protected $client;
 
@@ -87,7 +88,7 @@ public function __construct(string $api_key, string $api_secret, bool $debug = f
         $this->api_secret = $api_secret;
         $this->debug      = $debug;
 
-        // Specify a User-Agent for API requests.
+        // Set the Guzzle client.
         $this->client = new Client(
             [
                 'headers' => [
@@ -105,6 +106,20 @@ public function __construct(string $api_key, string $api_secret, bool $debug = f
         }
     }
 
+    /**
+     * Set the Guzzle client implementation to use for API requests.
+     *
+     * @param ClientInterface $client Guzzle client implementation.
+     *
+     * @since 1.3.0
+     *
+     * @return void
+     */
+    public function set_http_client(ClientInterface $client)
+    {
+        $this->client = $client;
+    }
+
     /**
      * Add an entry to monologger.
      *
diff --git a/tests/ConvertKitAPITest.php b/tests/ConvertKitAPITest.php
index 56483c2..d7ccfa5 100644
--- a/tests/ConvertKitAPITest.php
+++ b/tests/ConvertKitAPITest.php
@@ -1,6 +1,12 @@
 <?php
 
 use PHPUnit\Framework\TestCase;
+use GuzzleHttp\Client;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Psr7\Request;
+use GuzzleHttp\Psr7\Response;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\Exception\RequestException;
 
 /**
  * ConvertKit API class tests.
@@ -47,6 +53,42 @@ protected function setUp(): void
         $this->api = new \ConvertKit_API\ConvertKit_API($_ENV['CONVERTKIT_API_KEY'], $_ENV['CONVERTKIT_API_SECRET']);
     }
 
+    /**
+     * Test that a ClientInterface can be injected.
+     *
+     * @since   1.3.0
+     *
+     * @return  void
+     */
+    public function testClientInterfaceInjection()
+    {
+        // Setup API with a mock Guzzle client.
+        $mock = new MockHandler([
+            new Response(200, [], json_encode(
+                [
+                    'name' => 'Test Account for Guzzle Mock',
+                    'plan_type' => 'free',
+                    'primary_email_address' => 'mock@guzzle.mock',
+                ]
+            )),
+        ]);
+
+        // Define client with mock handler.
+        $handlerStack = HandlerStack::create($mock);
+        $client = new Client(['handler' => $handlerStack]);
+
+        // Assign the client to the API class.
+        $this->api->set_http_client($client);
+
+        // Perform an API request.
+        $result = $this->api->get_account();
+
+        // Confirm mocked data was returned.
+        $this->assertSame('Test Account for Guzzle Mock', $result->name);
+        $this->assertSame('free', $result->plan_type);
+        $this->assertSame('mock@guzzle.mock', $result->primary_email_address);
+    }
+
     /**
      * Test that debug logging works when enabled and an API call is made.
      *