diff --git a/src/wp-includes/class-wp-icon-collections-registry.php b/src/wp-includes/class-wp-icon-collections-registry.php
index e7e19d02b19e0..edd2b65ce18d3 100644
--- a/src/wp-includes/class-wp-icon-collections-registry.php
+++ b/src/wp-includes/class-wp-icon-collections-registry.php
@@ -154,6 +154,7 @@ public function register( $collection_slug, $collection_properties ) {
 	 * Unregisters an icon collection.
 	 *
 	 * Any icons registered under the given collection are also unregistered.
+	 * The core-admin collection cannot be unregistered.
 	 *
 	 * @since 7.1.0
 	 *
@@ -174,6 +175,15 @@ public function unregister( $collection_slug ) {
 			return false;
 		}
 
+		if ( 'core-admin' === $collection_slug ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'The "core-admin" icon collection cannot be unregistered.' ),
+				'7.2.0'
+			);
+			return false;
+		}
+
 		$icons_registry = WP_Icons_Registry::get_instance();
 		foreach ( $icons_registry->get_registered_icons() as $icon ) {
 			if ( isset( $icon['collection'] ) && $icon['collection'] === $collection_slug ) {
diff --git a/src/wp-includes/class-wp-icons-registry.php b/src/wp-includes/class-wp-icons-registry.php
index 6186a0b48907a..cbf747526619b 100644
--- a/src/wp-includes/class-wp-icons-registry.php
+++ b/src/wp-includes/class-wp-icons-registry.php
@@ -228,6 +228,8 @@ public function register( $icon_name, $icon_properties ) {
 	/**
 	 * Unregisters an icon.
 	 *
+	 * Icons in the core-admin collection cannot be unregistered.
+	 *
 	 * @since 7.1.0
 	 *
 	 * @param string $icon_name Namespaced icon name in the form "collection/icon-name"
@@ -248,6 +250,15 @@ public function unregister( $icon_name ) {
 			return false;
 		}
 
+		if ( 'core-admin' === $this->registered_icons[ $icon_name ]['collection'] ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'Icons in the "core-admin" collection cannot be unregistered.' ),
+				'7.2.0'
+			);
+			return false;
+		}
+
 		unset( $this->registered_icons[ $icon_name ] );
 		return true;
 	}
diff --git a/tests/phpunit/tests/icons/wpIconsRegistry.php b/tests/phpunit/tests/icons/wpIconsRegistry.php
index b647a5543b5ec..39643bdc6e052 100644
--- a/tests/phpunit/tests/icons/wpIconsRegistry.php
+++ b/tests/phpunit/tests/icons/wpIconsRegistry.php
@@ -479,6 +479,90 @@ public function test_unregister_unknown_icon() {
 		$this->assertFalse( $this->registry->unregister( 'test-collection/ghost' ) );
 	}
 
+	/**
+	 * @ticket 66344
+	 *
+	 * @covers ::unregister
+	 *
+	 * @expectedIncorrectUsage WP_Icons_Registry::unregister
+	 */
+	public function test_cannot_unregister_core_admin_icon() {
+		$this->registry->register(
+			'core-admin/test-icon',
+			array(
+				'label'   => 'Admin icon',
+				'content' => '<svg><path d="M0 0"/></svg>',
+			)
+		);
+		$before = wp_get_icon( 'core-admin/test-icon' );
+
+		$this->assertNotEmpty( $before );
+		$this->assertFalse( wp_unregister_icon( 'core-admin/test-icon' ) );
+		$this->assertSame( $before, wp_get_icon( 'core-admin/test-icon' ) );
+	}
+
+	/**
+	 * @ticket 66344
+	 *
+	 * @covers WP_Icon_Collections_Registry::unregister
+	 *
+	 * @expectedIncorrectUsage WP_Icon_Collections_Registry::unregister
+	 */
+	public function test_cannot_unregister_core_admin_collection() {
+		$this->registry->register(
+			'core-admin/test-icon',
+			array(
+				'label'   => 'Admin icon',
+				'content' => '<svg><path d="M0 0"/></svg>',
+			)
+		);
+		$collections = WP_Icon_Collections_Registry::get_instance();
+		$collection  = $collections->get_registered( 'core-admin' );
+		$before      = wp_get_icon( 'core-admin/test-icon' );
+
+		$this->assertNotEmpty( $before );
+		$this->assertFalse( wp_unregister_icon_collection( 'core-admin' ) );
+		$this->assertSame( $collection, $collections->get_registered( 'core-admin' ) );
+		$this->assertSame( $before, wp_get_icon( 'core-admin/test-icon' ) );
+	}
+
+	/**
+	 * @ticket 66344
+	 *
+	 * @covers ::unregister
+	 * @covers WP_Icon_Collections_Registry::unregister
+	 */
+	public function test_can_unregister_core_icons_and_collection() {
+		$collections = WP_Icon_Collections_Registry::get_instance();
+		$collection  = $collections->get_registered( 'core' );
+		unset( $collection['slug'] );
+
+		$this->registry->register(
+			'core/test-icon',
+			array(
+				'label'   => 'Core icon',
+				'content' => '<svg></svg>',
+			)
+		);
+		$this->registry->register(
+			'core/cascade-icon',
+			array(
+				'label'   => 'Core icon',
+				'content' => '<svg></svg>',
+			)
+		);
+
+		try {
+			$this->assertTrue( wp_unregister_icon( 'core/test-icon' ) );
+			$this->assertFalse( $this->registry->is_registered( 'core/test-icon' ) );
+			$this->assertTrue( wp_unregister_icon_collection( 'core' ) );
+			$this->assertFalse( $collections->is_registered( 'core' ) );
+			$this->assertFalse( $this->registry->is_registered( 'core/cascade-icon' ) );
+		} finally {
+			$collections->register( 'core', $collection );
+		}
+	}
+
 	/**
 	 * @ticket 64651
 	 *
diff --git a/tests/phpunit/tests/icons/wpRestIconsController.php b/tests/phpunit/tests/icons/wpRestIconsController.php
index 7a06a4a88846e..5909ff516087d 100644
--- a/tests/phpunit/tests/icons/wpRestIconsController.php
+++ b/tests/phpunit/tests/icons/wpRestIconsController.php
@@ -17,6 +17,13 @@ class Tests_REST_WpRestIconsController extends WP_Test_REST_Controller_Testcase
 	protected static $contributor_id;
 	protected static $subscriber_id;
 
+	/**
+	 * Registry instances restored after each test.
+	 *
+	 * @var array
+	 */
+	private $original_registries = array();
+
 	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
 		self::$admin_id       = $factory->user->create( array( 'role' => 'administrator' ) );
 		self::$editor_id      = $factory->user->create( array( 'role' => 'editor' ) );
@@ -34,17 +41,14 @@ public static function wpTearDownAfterClass() {
 	public function set_up() {
 		parent::set_up();
 
-		/*
-		 * Other suites reset the `WP_Icons_Registry` singleton, wiping the collections and
-		 * icons that `init` only registers once. Replay the registration so order-dependent
-		 * tests pass. `_wp_register_default_icon_collections()` registers every default
-		 * collection at once, so drop whatever survived rather than topping up.
-		 */
-		$collections_registry = WP_Icon_Collections_Registry::get_instance();
-		foreach ( array( 'core', 'core-admin' ) as $collection_slug ) {
-			if ( $collections_registry->is_registered( $collection_slug ) ) {
-				$collections_registry->unregister( $collection_slug );
+		// Use fresh registries, since the core-admin collection cannot be unregistered.
+		foreach ( array( WP_Icon_Collections_Registry::class, WP_Icons_Registry::class ) as $class ) {
+			$this->original_registries[ $class ] = $class::get_instance();
+			$instance_property                   = new ReflectionProperty( $class, 'instance' );
+			if ( PHP_VERSION_ID < 80100 ) {
+				$instance_property->setAccessible( true );
 			}
+			$instance_property->setValue( null, null );
 		}
 		_wp_register_default_icon_collections();
 		_wp_register_default_icons();
@@ -56,6 +60,14 @@ public function tear_down() {
 				wp_unregister_icon_collection( $slug );
 			}
 		}
+		foreach ( $this->original_registries as $class => $instance ) {
+			$instance_property = new ReflectionProperty( $class, 'instance' );
+			if ( PHP_VERSION_ID < 80100 ) {
+				$instance_property->setAccessible( true );
+			}
+			$instance_property->setValue( null, $instance );
+		}
+		$this->original_registries = array();
 		parent::tear_down();
 	}