diff --git a/integrations/ninja-forms/class-field.php b/integrations/ninja-forms/class-field.php
index ec8fcd22..adf3567f 100755
--- a/integrations/ninja-forms/class-field.php
+++ b/integrations/ninja-forms/class-field.php
@@ -62,6 +62,30 @@ public function admin_form_element($id, $value)
         return "<input type='hidden' name='fields[$id]' value='0' ><input type='checkbox' name='fields[$id]' value='1' id='' $checked>";
     }
 
+    /**
+     * Validate the field value.
+     *
+     * Overrides parent to handle checkbox-specific required validation,
+     * since checkboxes submit '0' when unchecked which the parent considers valid.
+     *
+     * @since 4.9
+     *
+     * @param array $field The field data.
+     * @param array $data  The form data.
+     * @return array Array of validation errors, empty if valid.
+     */
+    public function validate($field, $data)
+    {
+        $errors = parent::validate($field, $data);
+
+        if (isset($field['required']) && 1 == intval($field['required']) && empty($field['value'])) {
+            $errors['slug']    = 'required-error';
+            $errors['message'] = esc_html__('This field is required.', 'mailchimp-for-wp');
+        }
+
+        return $errors;
+    }
+
     /**
     * Custom Columns
     * Creates what is displayed in the columns on the submissions page.
diff --git a/tests/NinjaFormsValidationTest.php b/tests/NinjaFormsValidationTest.php
new file mode 100644
index 00000000..d056afdb
--- /dev/null
+++ b/tests/NinjaFormsValidationTest.php
@@ -0,0 +1,100 @@
+<?php
+
+// phpcs:disable PSR1.Classes.ClassDeclaration.MultipleClasses
+
+// Mock WP functions
+if (! function_exists('add_filter')) {
+    function add_filter()
+    {
+    }
+}
+
+if (! function_exists('add_action')) {
+    function add_action()
+    {
+    }
+}
+
+if (! function_exists('__')) {
+    function __($text, $domain = 'default')
+    {
+        return $text;
+    }
+}
+
+if (! function_exists('esc_html__')) {
+    function esc_html__($text, $domain = 'default')
+    {
+        return $text;
+    }
+}
+
+// Mock Parent Class
+if (! class_exists('NF_Abstracts_Input')) {
+    abstract class NF_Abstracts_Input
+    {
+        protected $_settings = []; // Needed for child constructor access
+
+        public function __construct()
+        {
+            // Mock constructor
+        }
+
+        public function validate($field, $data)
+        {
+            return [];
+        }
+    }
+}
+
+require_once __DIR__ . '/../integrations/ninja-forms/class-field.php';
+
+class NinjaFormsValidationTest extends \PHPUnit\Framework\TestCase
+{
+    public function testRequiredValidationFailsOnEmpty()
+    {
+        $field = new MC4WP_Ninja_Forms_Field();
+
+        $field_data = [
+            'required' => 1,
+            'value' => 0,
+        ];
+        $form_data = [];
+
+        $errors = $field->validate($field_data, $form_data);
+
+        $this->assertIsArray($errors);
+        $this->assertArrayHasKey('slug', $errors, 'Error slug should be present');
+        $this->assertEquals('required-error', $errors['slug']);
+    }
+
+    public function testRequiredValidationPassesOnChecked()
+    {
+        $field = new MC4WP_Ninja_Forms_Field();
+
+        $field_data = [
+            'required' => 1,
+            'value' => 1,
+        ];
+        $form_data = [];
+
+        $errors = $field->validate($field_data, $form_data);
+
+        $this->assertEmpty($errors, 'Validation should pass for required checked field');
+    }
+
+    public function testOptionalValidationPassesOnUnchecked()
+    {
+        $field = new MC4WP_Ninja_Forms_Field();
+
+        $field_data = [
+            'required' => 0,
+            'value' => 0,
+        ];
+        $form_data = [];
+
+        $errors = $field->validate($field_data, $form_data);
+
+        $this->assertEmpty($errors, 'Validation should pass for optional unchecked field');
+    }
+}