diff --git a/forms/GroupedDropdownField.php b/forms/GroupedDropdownField.php
index 500ecfa9e..f128d94ac 100644
--- a/forms/GroupedDropdownField.php
+++ b/forms/GroupedDropdownField.php
@@ -89,11 +89,42 @@ class GroupedDropdownField extends DropdownField {
 	}
 
 	/**
-	 * @todo Implement DropdownField::validate() with group validation support
+	 * Validate this field
+	 *
+	 * @param Validator $validator
+	 * @return bool
 	 */
 	public function validate($validator) {
+		$valid = false;
+		$source = $this->getSourceAsArray();
+
+		if ($this->value) {
+			foreach ($source as $value => $title) {
+				// Check if value matches an option, or is in a nested array of grouped options
+				if ((is_array($title) && array_key_exists($this->value, $title))
+					|| ($this->value == $value)
+				) {
+					$valid = true;
+				}
+			}
+		} elseif ($this->getHasEmptyDefault()) {
+			$valid = true;
+		}
+
+		if (!$valid) {
+			$validator->validationError(
+				$this->name,
+				_t(
+					'DropdownField.SOURCE_VALIDATION',
+					"Please select a value within the list provided. {value} is not a valid option",
+					array('value' => $this->value)
+				),
+				"validation"
+			);
+			return false;
+		}
+
 		return true;
 	}
 
 }
-
diff --git a/tests/forms/GroupedDropdownFieldTest.php b/tests/forms/GroupedDropdownFieldTest.php
new file mode 100644
index 000000000..772684f91
--- /dev/null
+++ b/tests/forms/GroupedDropdownFieldTest.php
@@ -0,0 +1,44 @@
+<?php
+
+/**
+ * @package framework
+ * @subpackage tests
+ */
+class GroupedDropdownFieldTest extends SapphireTest {
+
+	public function testValidation() {
+		$field = GroupedDropdownField::create('Test', 'Testing', array(
+			"1" => "One",
+			"Group One" => array(
+				"2" => "Two",
+				"3" => "Three"
+			),
+			"Group Two" => array(
+				"4" => "Four"
+			)
+		));
+
+		$validator = new RequiredFields();
+
+		$field->setValue("1");
+		$this->assertTrue($field->validate($validator));
+
+		//test grouped values
+		$field->setValue("3");
+		$this->assertTrue($field->validate($validator));
+
+		//non-existent value should make the field invalid
+		$field->setValue("Over 9000");
+		$this->assertFalse($field->validate($validator));
+
+		//empty string shouldn't validate
+		$field->setValue('');
+		$this->assertFalse($field->validate($validator));
+
+		//empty field should validate after being set
+		$field->setEmptyString('Empty String');
+		$field->setValue('');
+		$this->assertTrue($field->validate($validator));
+	}
+
+}