diff --git a/CHANGES.md b/CHANGES.md
index 3b02f5e..4a430c6 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,10 @@
+## 0.12.5
+
+- Added Rent Fetch Sync API key setting with asynchronous validation
+- API key field includes a separate "Validate Key" button for testing without saving
+- Automatic validation on page load when an API key is present
+- Handles multiple validation states: valid, not found, and already in use
+
 ## 0.12.4
 
 - Updating hooks to match the new RF pattern for subpages in the General settings.
diff --git a/assets/js/rentfetch-rfs-api-key-validation.js b/assets/js/rentfetch-rfs-api-key-validation.js
new file mode 100644
index 0000000..f6cd06d
--- /dev/null
+++ b/assets/js/rentfetch-rfs-api-key-validation.js
@@ -0,0 +1,92 @@
+jQuery(document).ready(function ($) {
+	var $apiKeyInput = $('#rentfetch_options_rfs_api_key');
+	var $validateButton = $('#rfs_validate_api_key');
+	var $statusDiv = $('#rfs_api_key_status');
+
+	/**
+	 * Display a status message
+	 *
+	 * @param {string} message The message to display
+	 * @param {string} type The type of message: 'success', 'error', 'warning', or 'info'
+	 */
+	function displayStatus(message, type) {
+		var cssClass = 'notice notice-' + type;
+		$statusDiv
+			.html('<div class="' + cssClass + '"><p>' + message + '</p></div>')
+			.show();
+	}
+
+	/**
+	 * Validate the API key
+	 *
+	 * @param {boolean} showLoadingState Whether to show a loading state on the button
+	 */
+	function validateApiKey(showLoadingState) {
+		var apiKey = $apiKeyInput.val().trim();
+
+		if (!apiKey) {
+			displayStatus('Please enter an API key.', 'warning');
+			return;
+		}
+
+		if (showLoadingState) {
+			$validateButton.prop('disabled', true).text('Validating...');
+		}
+
+		$.post(rfs_ajax_object.ajax_url, {
+			action: 'rfs_validate_api_key',
+			api_key: apiKey,
+			_ajax_nonce: rfs_ajax_object.nonce, // Must match parameter name in PHP: check_ajax_referer('rfs_ajax_nonce', '_ajax_nonce')
+		})
+			.done(function (response) {
+				if (response.success) {
+					var data = response.data;
+					var status = data.status;
+					var message = data.message;
+
+					switch (status) {
+						case 'valid':
+							displayStatus(message, 'success');
+							break;
+						case 'not_found':
+							displayStatus(message, 'error');
+							break;
+						case 'in_use':
+							displayStatus(message, 'warning');
+							break;
+						default:
+							displayStatus(message, 'info');
+							break;
+					}
+				} else {
+					displayStatus(
+						'Error validating API key: ' +
+							(response.data || 'Unknown error'),
+						'error'
+					);
+				}
+			})
+			.fail(function (xhr, status, error) {
+				displayStatus(
+					'Network error while validating API key: ' + error,
+					'error'
+				);
+			})
+			.always(function () {
+				if (showLoadingState) {
+					$validateButton.prop('disabled', false).text('Validate Key');
+				}
+			});
+	}
+
+	// Validate on button click
+	$validateButton.on('click', function (e) {
+		e.preventDefault();
+		validateApiKey(true);
+	});
+
+	// Validate on page load if API key exists
+	if ($apiKeyInput.length && $apiKeyInput.val().trim()) {
+		validateApiKey(false);
+	}
+});
diff --git a/lib/admin/ajax-validate-rfs-api-key.php b/lib/admin/ajax-validate-rfs-api-key.php
new file mode 100644
index 0000000..2eae995
--- /dev/null
+++ b/lib/admin/ajax-validate-rfs-api-key.php
@@ -0,0 +1,140 @@
+<?php
+/**
+ * AJAX handler for RFS API key validation
+ *
+ * @package rentfetch-sync
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit; // Exit if accessed directly.
+}
+
+/**
+ * Validate the RFS API key against the Rent Fetch API
+ *
+ * @return void
+ */
+function rfs_validate_api_key_ajax_handler() {
+	// Verify nonce
+	check_ajax_referer( 'rfs_ajax_nonce', '_ajax_nonce' );
+
+	// Check if API key is provided
+	if ( ! isset( $_POST['api_key'] ) ) {
+		wp_send_json_error( 'API key is required.' );
+	}
+
+	$api_key = sanitize_text_field( wp_unslash( $_POST['api_key'] ) );
+
+	if ( empty( $api_key ) ) {
+		wp_send_json_error( 'API key cannot be empty.' );
+	}
+
+	// Validate the API key against the Rent Fetch API
+	$validation_result = rfs_validate_api_key_with_api( $api_key );
+
+	if ( is_wp_error( $validation_result ) ) {
+		wp_send_json_error( $validation_result->get_error_message() );
+	}
+
+	wp_send_json_success( $validation_result );
+}
+add_action( 'wp_ajax_rfs_validate_api_key', 'rfs_validate_api_key_ajax_handler' );
+
+/**
+ * Validate the API key with the Rent Fetch API
+ *
+ * @param string $api_key The API key to validate.
+ *
+ * @return array|WP_Error The validation result or WP_Error on failure.
+ */
+function rfs_validate_api_key_with_api( $api_key ) {
+	
+	// Get the site URL for validation
+	$site_url = get_site_url();
+	
+	// Extract just the domain name (host) from the URL
+	$parsed_url = wp_parse_url( $site_url );
+	if ( is_array( $parsed_url ) && isset( $parsed_url['host'] ) ) {
+		$site_url = $parsed_url['host'];
+	}
+
+	// Prepare the request body
+	$body = array(
+		'api_key'  => $api_key,
+		'site_url' => $site_url,
+	);
+
+	// Get the API URL (allow override via constant for testing)
+	$api_url = defined( 'RENTFETCH_API_VALIDATE_URL' ) 
+		? constant( 'RENTFETCH_API_VALIDATE_URL' ) 
+		: 'https://api.rentfetch.net/wp-json/rentfetchapi/v1/validate-key';
+
+	// Make the API request
+	$response = wp_remote_post(
+		$api_url,
+		array(
+			'method'  => 'POST',
+			'body'    => wp_json_encode( $body ),
+			'headers' => array(
+				'Content-Type' => 'application/json',
+			),
+			'timeout' => 15,
+		)
+	);
+
+	// Check for errors
+	if ( is_wp_error( $response ) ) {
+		return new WP_Error( 
+			'api_request_failed', 
+			'Unable to connect to Rent Fetch API: ' . $response->get_error_message() 
+		);
+	}
+
+	$response_code = wp_remote_retrieve_response_code( $response );
+	$response_body = wp_remote_retrieve_body( $response );
+
+	// Parse the response
+	$data = json_decode( $response_body, true );
+
+	// Handle different response codes
+	switch ( $response_code ) {
+		case 200:
+			// Valid API key
+			return array(
+				'status'  => 'valid',
+				'message' => isset( $data['message'] ) 
+					? $data['message'] 
+					: 'API key is valid and ready to use.',
+			);
+
+		case 404:
+			// API key not found
+			return array(
+				'status'  => 'not_found',
+				'message' => isset( $data['message'] ) 
+					? $data['message'] 
+					: 'API key not found. Please check your key and try again.',
+			);
+
+		case 409:
+			// API key already in use on another site
+			$existing_site = isset( $data['existing_site'] ) ? $data['existing_site'] : 'another site';
+			return array(
+				'status'  => 'in_use',
+				'message' => isset( $data['message'] ) 
+					? $data['message'] 
+					: sprintf( 'This API key is already in use on %s.', $existing_site ),
+			);
+
+		default:
+			// Unexpected response
+			return new WP_Error(
+				'unexpected_response',
+				sprintf(
+					'Unexpected response from API (HTTP %d): %s',
+					$response_code,
+					isset( $data['message'] ) ? $data['message'] : wp_remote_retrieve_response_message( $response )
+				)
+			);
+	}
+}
diff --git a/lib/admin/sync-options.php b/lib/admin/sync-options.php
index 413e9bc..377f39d 100644
--- a/lib/admin/sync-options.php
+++ b/lib/admin/sync-options.php
@@ -13,6 +13,20 @@ function rentfetch_remove_premium_functionality_notice() {
  */
 add_action( 'rentfetch_do_settings_general_data_sync', 'rentfetch_settings_sync', 25 );
 function rentfetch_settings_sync() {
+	
+	// Enqueue the API key validation script
+	wp_enqueue_script( 'rentfetch-rfs-api-key-validation' );
+	
+	// Localize script to pass AJAX URL and nonce
+	wp_localize_script(
+		'rentfetch-rfs-api-key-validation',
+		'rfs_ajax_object',
+		array(
+			'ajax_url' => admin_url( 'admin-ajax.php' ),
+			'nonce'    => wp_create_nonce( 'rfs_ajax_nonce' ),
+		)
+	);
+	
 	?>
 	<div class="header">
 		<h2 class="title">Data Sync</h2>
@@ -46,6 +60,21 @@ function rentfetch_settings_sync() {
 				</ul>
 			</div>
 		</div>
+
+		<div class="row">
+			<div class="section">
+				<label class="label-large" for="rentfetch_options_rfs_api_key">Rent Fetch Sync API Key</label>
+				<p class="description">Enter your Rent Fetch Sync API key to enable synchronization features.</p>
+				<div class="white-box">
+					<div style="margin-bottom: 10px;">
+						<input type="password" name="rentfetch_options_rfs_api_key" id="rentfetch_options_rfs_api_key" value="<?php echo esc_attr( get_option( 'rentfetch_options_rfs_api_key' ) ); ?>" style="width: 100%; max-width: 400px;">
+						<button type="button" id="rfs_validate_api_key" class="button" style="margin-left: 10px;">Validate Key</button>
+					</div>
+					<div id="rfs_api_key_status"></div>
+				</div>
+			</div>
+		</div>
+		
 		<style>
 			.integration-settings { display: none; }
 			label:has(input[name="rentfetch_options_enabled_integrations[]"]:checked) ~ .integration-settings { display: block; }
diff --git a/lib/initialization/enqueue.php b/lib/initialization/enqueue.php
index 531adec..48fd2e9 100644
--- a/lib/initialization/enqueue.php
+++ b/lib/initialization/enqueue.php
@@ -22,6 +22,14 @@ function rfs_enqueue_backend_scripts() {
 		RENTFETCHSYNC_VERSION,
 		true
 	);
+	
+	wp_register_script(
+		'rentfetch-rfs-api-key-validation',
+		RENTFETCHSYNC_PATH . 'assets/js/rentfetch-rfs-api-key-validation.js',
+		array( 'jquery' ),
+		RENTFETCHSYNC_VERSION,
+		true
+	);
 }
 add_action( 'admin_enqueue_scripts', 'rfs_enqueue_backend_scripts' );
 
diff --git a/rentfetch-sync.php b/rentfetch-sync.php
index 9006452..7a0c0b5 100644
--- a/rentfetch-sync.php
+++ b/rentfetch-sync.php
@@ -3,7 +3,7 @@
 	Plugin Name:   Rent Fetch Sync
 	Plugin URI:    https://github.com/jonschr/rentfetch-sync
 	Description:   An addon for Rent Fetch that syncs properties, floorplans, and units
-	Version:       0.12.4
+	Version:       0.12.5
 	Author:        Brindle Digital
 	Author URI:    https://www.brindledigital.com/
 
@@ -24,7 +24,7 @@
 }
 
 // Define the version of the plugin.
-define( 'RENTFETCHSYNC_VERSION', '0.12.4' );
+define( 'RENTFETCHSYNC_VERSION', '0.12.5' );
 
 // Plugin directory.
 define( 'RENTFETCHSYNC_DIR', plugin_dir_path( __FILE__ ) );