diff --git a/changelog/fix-affirm-domestic-transactions b/changelog/fix-affirm-domestic-transactions
new file mode 100644
index 00000000000..52c2a7ca373
--- /dev/null
+++ b/changelog/fix-affirm-domestic-transactions
@@ -0,0 +1,4 @@
+Significance: patch
+Type: update
+
+update: payment method definition to determine eligible countries
diff --git a/includes/payment-methods/Configs/Definitions/AffirmDefinition.php b/includes/payment-methods/Configs/Definitions/AffirmDefinition.php
index 64c2ebb45b3..0517dee0e88 100644
--- a/includes/payment-methods/Configs/Definitions/AffirmDefinition.php
+++ b/includes/payment-methods/Configs/Definitions/AffirmDefinition.php
@@ -90,15 +90,26 @@ public static function get_supported_currencies(): array {
 	}
 
 	/**
-	 * Get the list of supported countries
+	 * Get the list of supported countries.
 	 *
+	 * Affirm only supports domestic transactions, so when account_country is provided,
+	 * only that country is returned if it's in the supported list.
+	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
-		return [
+	public static function get_supported_countries( ?string $account_country = null ): array {
+		$supported_countries = [
 			Country_Code::UNITED_STATES,
 			Country_Code::CANADA,
 		];
+
+		// Affirm only supports domestic transactions.
+		if ( null !== $account_country && in_array( strtoupper( $account_country ), $supported_countries, true ) ) {
+			return [ strtoupper( $account_country ) ];
+		}
+
+		return $supported_countries;
 	}
 
 	/**
@@ -188,7 +199,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country );
+		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country );
 	}
 
 	/**
diff --git a/includes/payment-methods/Configs/Definitions/AlipayDefinition.php b/includes/payment-methods/Configs/Definitions/AlipayDefinition.php
index 6d20d5aa65b..e6356843309 100644
--- a/includes/payment-methods/Configs/Definitions/AlipayDefinition.php
+++ b/includes/payment-methods/Configs/Definitions/AlipayDefinition.php
@@ -165,9 +165,10 @@ public static function get_supported_currencies(): array {
 	/**
 	 * Get the list of supported countries
 	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [];
 	}
 
@@ -244,7 +245,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country );
+		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country );
 	}
 
 	/**
diff --git a/includes/payment-methods/Configs/Definitions/ApplePayDefinition.php b/includes/payment-methods/Configs/Definitions/ApplePayDefinition.php
index 6a70d410c63..9d652561040 100644
--- a/includes/payment-methods/Configs/Definitions/ApplePayDefinition.php
+++ b/includes/payment-methods/Configs/Definitions/ApplePayDefinition.php
@@ -89,9 +89,10 @@ public static function get_supported_currencies(): array {
 	/**
 	 * Get the list of supported countries
 	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [];
 	}
 
@@ -170,7 +171,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		if ( ! PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country ) ) {
+		if ( ! PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country ) ) {
 			return false;
 		}
 
diff --git a/includes/payment-methods/Configs/Definitions/BancontactDefinition.php b/includes/payment-methods/Configs/Definitions/BancontactDefinition.php
index 09c185ee6fa..63c228a7a5a 100644
--- a/includes/payment-methods/Configs/Definitions/BancontactDefinition.php
+++ b/includes/payment-methods/Configs/Definitions/BancontactDefinition.php
@@ -89,9 +89,10 @@ public static function get_supported_currencies(): array {
 	/**
 	 * Get the list of supported countries
 	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [ Country_Code::BELGIUM ];
 	}
 
@@ -167,7 +168,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country );
+		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country );
 	}
 
 	/**
diff --git a/includes/payment-methods/Configs/Definitions/EpsDefinition.php b/includes/payment-methods/Configs/Definitions/EpsDefinition.php
index 3e8bedfe5f8..7b43dd34a89 100644
--- a/includes/payment-methods/Configs/Definitions/EpsDefinition.php
+++ b/includes/payment-methods/Configs/Definitions/EpsDefinition.php
@@ -89,9 +89,10 @@ public static function get_supported_currencies(): array {
 	/**
 	 * Get the list of supported countries
 	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [ Country_Code::AUSTRIA ];
 	}
 
@@ -167,7 +168,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country );
+		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country );
 	}
 
 	/**
diff --git a/includes/payment-methods/Configs/Definitions/GiropayDefinition.php b/includes/payment-methods/Configs/Definitions/GiropayDefinition.php
index e7d7fde56e3..a74786f4b0a 100644
--- a/includes/payment-methods/Configs/Definitions/GiropayDefinition.php
+++ b/includes/payment-methods/Configs/Definitions/GiropayDefinition.php
@@ -89,9 +89,10 @@ public static function get_supported_currencies(): array {
 	/**
 	 * Get the list of supported countries
 	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [ Country_Code::GERMANY ];
 	}
 
@@ -168,7 +169,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country );
+		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country );
 	}
 
 	/**
diff --git a/includes/payment-methods/Configs/Definitions/GooglePayDefinition.php b/includes/payment-methods/Configs/Definitions/GooglePayDefinition.php
index f90c3fa8eb2..8415f28fca1 100644
--- a/includes/payment-methods/Configs/Definitions/GooglePayDefinition.php
+++ b/includes/payment-methods/Configs/Definitions/GooglePayDefinition.php
@@ -89,9 +89,10 @@ public static function get_supported_currencies(): array {
 	/**
 	 * Get the list of supported countries
 	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [];
 	}
 
@@ -170,7 +171,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		if ( ! PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country ) ) {
+		if ( ! PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country ) ) {
 			return false;
 		}
 
diff --git a/includes/payment-methods/Configs/Definitions/IdealDefinition.php b/includes/payment-methods/Configs/Definitions/IdealDefinition.php
index 2ea60db1473..1b73f1b50eb 100644
--- a/includes/payment-methods/Configs/Definitions/IdealDefinition.php
+++ b/includes/payment-methods/Configs/Definitions/IdealDefinition.php
@@ -89,9 +89,10 @@ public static function get_supported_currencies(): array {
 	/**
 	 * Get the list of supported countries
 	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [ Country_Code::NETHERLANDS ];
 	}
 
@@ -168,7 +169,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country );
+		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country );
 	}
 
 	/**
diff --git a/includes/payment-methods/Configs/Definitions/P24Definition.php b/includes/payment-methods/Configs/Definitions/P24Definition.php
index bb6ca4b4a44..81ed4dd2c90 100644
--- a/includes/payment-methods/Configs/Definitions/P24Definition.php
+++ b/includes/payment-methods/Configs/Definitions/P24Definition.php
@@ -92,9 +92,10 @@ public static function get_supported_currencies(): array {
 	/**
 	 * Get the list of supported countries
 	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [ Country_Code::POLAND ];
 	}
 
@@ -170,7 +171,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country );
+		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country );
 	}
 
 	/**
diff --git a/includes/payment-methods/Configs/Definitions/SofortDefinition.php b/includes/payment-methods/Configs/Definitions/SofortDefinition.php
index b600fc53928..9e9c2005b5e 100644
--- a/includes/payment-methods/Configs/Definitions/SofortDefinition.php
+++ b/includes/payment-methods/Configs/Definitions/SofortDefinition.php
@@ -89,9 +89,10 @@ public static function get_supported_currencies(): array {
 	/**
 	 * Get the list of supported countries
 	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [
 			Country_Code::AUSTRIA,
 			Country_Code::BELGIUM,
@@ -173,7 +174,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country );
+		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country );
 	}
 
 	/**
diff --git a/includes/payment-methods/Configs/Definitions/WechatPayDefinition.php b/includes/payment-methods/Configs/Definitions/WechatPayDefinition.php
index 1410499075d..d9ff74b286b 100644
--- a/includes/payment-methods/Configs/Definitions/WechatPayDefinition.php
+++ b/includes/payment-methods/Configs/Definitions/WechatPayDefinition.php
@@ -157,9 +157,10 @@ public static function get_supported_currencies(): array {
 	/**
 	 * Get the list of supported countries
 	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [
 			Country_Code::UNITED_STATES,
 			Country_Code::AUSTRALIA,
@@ -259,7 +260,7 @@ public static function get_limits_per_currency(): array {
 	 * @return bool
 	 */
 	public static function is_available_for( string $currency, string $account_country ): bool {
-		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries(), $currency, $account_country );
+		return PaymentMethodUtils::is_available_for( self::get_supported_currencies(), self::get_supported_countries( $account_country ), $currency, $account_country );
 	}
 
 	/**
diff --git a/includes/payment-methods/Configs/Interfaces/PaymentMethodDefinitionInterface.php b/includes/payment-methods/Configs/Interfaces/PaymentMethodDefinitionInterface.php
index abb2424d222..14ef4700437 100644
--- a/includes/payment-methods/Configs/Interfaces/PaymentMethodDefinitionInterface.php
+++ b/includes/payment-methods/Configs/Interfaces/PaymentMethodDefinitionInterface.php
@@ -69,9 +69,14 @@ public static function get_supported_currencies(): array;
 	 * Get the list of supported countries
 	 * Empty array means all countries are supported
 	 *
+	 * When account_country is provided, payment methods with domestic transaction
+	 * restrictions should return only that country (if supported), enabling
+	 * proper filtering at checkout.
+	 *
+	 * @param string|null $account_country Optional. The merchant's account country.
 	 * @return string[] Array of country codes
 	 */
-	public static function get_supported_countries(): array;
+	public static function get_supported_countries( ?string $account_country = null ): array;
 
 	/**
 	 * Get the payment method capabilities
diff --git a/includes/payment-methods/class-upe-payment-method.php b/includes/payment-methods/class-upe-payment-method.php
index 28722c1318a..afd7cbb85f6 100644
--- a/includes/payment-methods/class-upe-payment-method.php
+++ b/includes/payment-methods/class-upe-payment-method.php
@@ -385,6 +385,10 @@ public function get_countries() {
 		$account         = \WC_Payments::get_account_service()->get_cached_account_data();
 		$account_country = isset( $account['country'] ) ? strtoupper( $account['country'] ) : '';
 
+		if ( ! empty( $this->definition ) ) {
+			return $this->definition::get_supported_countries( $account_country );
+		}
+
 		return $this->has_domestic_transactions_restrictions() ? [ $account_country ] : $this->countries;
 	}
 
diff --git a/tests/unit/payment-methods/Configs/mocks/class-mock-payment-method-definition-two.php b/tests/unit/payment-methods/Configs/mocks/class-mock-payment-method-definition-two.php
index af1cea55d7b..3dccd0a9ac5 100644
--- a/tests/unit/payment-methods/Configs/mocks/class-mock-payment-method-definition-two.php
+++ b/tests/unit/payment-methods/Configs/mocks/class-mock-payment-method-definition-two.php
@@ -42,7 +42,7 @@ public static function get_description( ?string $account_country = null ): strin
 		return 'Second mock payment method for testing';
 	}
 
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [ 'US' ];
 	}
 
@@ -72,7 +72,7 @@ public static function get_testing_instructions( string $account_country ): stri
 
 	public static function is_available_for( string $currency, string $account_country ): bool {
 		return in_array( $currency, self::get_supported_currencies(), true ) &&
-			in_array( $account_country, self::get_supported_countries(), true );
+			in_array( $account_country, self::get_supported_countries( $account_country ), true );
 	}
 
 	public static function get_limits_per_currency(): array {
diff --git a/tests/unit/payment-methods/Configs/mocks/class-mock-payment-method-definition.php b/tests/unit/payment-methods/Configs/mocks/class-mock-payment-method-definition.php
index 5c490b11aee..c8f7677e22b 100644
--- a/tests/unit/payment-methods/Configs/mocks/class-mock-payment-method-definition.php
+++ b/tests/unit/payment-methods/Configs/mocks/class-mock-payment-method-definition.php
@@ -46,7 +46,7 @@ public static function get_supported_currencies(): array {
 		return [ 'USD', 'CAD' ];
 	}
 
-	public static function get_supported_countries(): array {
+	public static function get_supported_countries( ?string $account_country = null ): array {
 		return [ 'US', 'CA' ];
 	}
 
@@ -72,7 +72,7 @@ public static function get_testing_instructions( string $account_country ): stri
 
 	public static function is_available_for( string $currency, string $account_country ): bool {
 		return in_array( $currency, self::get_supported_currencies(), true ) &&
-			in_array( $account_country, self::get_supported_countries(), true );
+			in_array( $account_country, self::get_supported_countries( $account_country ), true );
 	}
 
 	public static function get_limits_per_currency(): array {