diff --git a/src/wp-includes/class-wp-icons-registry.php b/src/wp-includes/class-wp-icons-registry.php
index 2e306bec77e53..ceec97b50e665 100644
--- a/src/wp-includes/class-wp-icons-registry.php
+++ b/src/wp-includes/class-wp-icons-registry.php
@@ -111,6 +111,34 @@ protected function register( $icon_name, $icon_properties ) {
 			return false;
 		}
 
+		if ( preg_match( '/[A-Z]/', $icon_name ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'Icon names must not contain uppercase characters.' ),
+				'7.1.0'
+			);
+			return false;
+		}
+
+		$name_matcher = '/^[a-z][a-z0-9-]*\/[a-z][a-z0-9-]*$/';
+		if ( ! preg_match( $name_matcher, $icon_name ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'Icon names must contain a namespace prefix. Example: my-plugin/my-custom-icon' ),
+				'7.1.0'
+			);
+			return false;
+		}
+
+		if ( $this->is_registered( $icon_name ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'Icon is already registered.' ),
+				'7.1.0'
+			);
+			return false;
+		}
+
 		$allowed_keys = array_fill_keys( array( 'label', 'content', 'filePath' ), 1 );
 		foreach ( array_keys( $icon_properties ) as $key ) {
 			if ( ! array_key_exists( $key, $allowed_keys ) ) {
diff --git a/tests/phpunit/tests/icons/wpIconsRegistry.php b/tests/phpunit/tests/icons/wpIconsRegistry.php
new file mode 100644
index 0000000000000..5cfc6df48101e
--- /dev/null
+++ b/tests/phpunit/tests/icons/wpIconsRegistry.php
@@ -0,0 +1,118 @@
+<?php
+/**
+ * Tests for WP_Icons_Registry::register().
+ *
+ * @package WordPress
+ * @subpackage Icons
+ *
+ * @group icons
+ * @covers WP_Icons_Registry::register
+ * @covers WP_Icons_Registry::is_registered
+ */
+class Tests_Icons_WpIconsRegistry extends WP_UnitTestCase {
+
+	/**
+	 * Registry instance for testing.
+	 *
+	 * @var WP_Icons_Registry
+	 */
+	private $registry;
+
+	public function set_up() {
+		parent::set_up();
+		$this->registry = WP_Icons_Registry::get_instance();
+	}
+
+	public function tear_down() {
+		$instance_property = new ReflectionProperty( WP_Icons_Registry::class, 'instance' );
+
+		/*
+		 * ReflectionProperty::setAccessible is:
+		 * - redundant as of 8.1.0, which made all properties accessible
+		 * - deprecated as of 8.5.0
+		 * - needed until 8.1.0, as property `instance` is private
+		 */
+		if ( PHP_VERSION_ID < 80100 ) {
+			$instance_property->setAccessible( true );
+		}
+
+		$instance_property->setValue( null, null );
+
+		$this->registry = null;
+		parent::tear_down();
+	}
+
+	/**
+	 * Invokes WP_Icons_Registry::register despite it being private
+	 *
+	 * @param string $icon_name       Icon name including namespace.
+	 * @param array  $icon_properties Icon properties (label, content, filePath).
+	 * @return bool True if the icon was registered successfully.
+	 */
+	private function register( $icon_name, $icon_properties ) {
+		$method = new ReflectionMethod( $this->registry, 'register' );
+
+		/*
+		 * ReflectionMethod::setAccessible is:
+		 * - redundant as of 8.1.0, which made all properties accessible
+		 * - deprecated as of 8.5.0
+		 * - needed until 8.1.0, as property `instance` is private
+		 */
+		if ( PHP_VERSION_ID < 80100 ) {
+			$method->setAccessible( true );
+		}
+
+		return $method->invoke( $this->registry, $icon_name, $icon_properties );
+	}
+
+	/**
+	 * Provides invalid icon names.
+	 *
+	 * @return array[]
+	 */
+	public function data_invalid_icon_names() {
+		return array(
+			'non-string name'      => array( 1 ),
+			'no namespace'         => array( 'plus' ),
+			'uppercase characters' => array( 'Test/Plus' ),
+			'invalid characters'   => array( 'test/_doing_it_wrong' ),
+		);
+	}
+
+	/**
+	 * Should fail to re-register the same icon.
+	 *
+	 * @expectedIncorrectUsage WP_Icons_Registry::register
+	 */
+	public function test_register_icon_twice() {
+		$name     = 'test-plugin/duplicate';
+		$settings = array(
+			'label'   => 'Icon',
+			'content' => '<svg></svg>',
+		);
+
+		$result = $this->register( $name, $settings );
+		$this->assertTrue( $result );
+		$result2 = $this->register( $name, $settings );
+		$this->assertFalse( $result2 );
+	}
+
+
+	/**
+	 * Should fail to register icon with invalid names.
+	 *
+	 * @dataProvider data_invalid_icon_names
+	 * @expectedIncorrectUsage WP_Icons_Registry::register
+	 */
+	public function test_register_invalid_name() {
+		foreach ( $this->data_invalid_icon_names() as $name ) {
+			$settings = array(
+				'label'   => 'Icon',
+				'content' => '<svg></svg>',
+			);
+
+			$result = $this->register( $name, $settings );
+			$this->assertFalse( $result );
+		}
+	}
+}