diff --git a/src/wp-includes/class-wp-icons-registry.php b/src/wp-includes/class-wp-icons-registry.php
index 1851a4150a057..4d2288a0a2218 100644
--- a/src/wp-includes/class-wp-icons-registry.php
+++ b/src/wp-includes/class-wp-icons-registry.php
@@ -20,6 +20,14 @@ class WP_Icons_Registry {
 	 */
 	protected $registered_icons = array();
 
+	/**
+	 * Whether the default icons bundled with core have been registered yet.
+	 *
+	 * @since 7.1.0
+	 * @var bool
+	 */
+	protected $default_icons_loaded = false;
+
 	/**
 	 * Container for the main instance of the class.
 	 *
@@ -336,6 +344,8 @@ public function get_registered_icon( $icon_name ) {
 	 * @return array[] Array of arrays containing the registered icon properties.
 	 */
 	public function get_registered_icons( $search = '' ) {
+		$this->load_default_icons();
+
 		$icons = array();
 
 		foreach ( $this->registered_icons as $icon ) {
@@ -362,9 +372,35 @@ public function get_registered_icons( $search = '' ) {
 	 * @return bool True if the icon is registered, false otherwise.
 	 */
 	public function is_registered( $icon_name ) {
+		$this->load_default_icons();
+
 		return isset( $this->registered_icons[ $icon_name ] );
 	}
 
+	/**
+	 * Registers the default icons bundled with core, once, on first use.
+	 *
+	 * The core icon manifest declares 88 icons, each with a translated label.
+	 * Registering them eagerly costs every request — including front-end
+	 * requests that never render an icon — so registration is deferred until
+	 * something actually reads the registry.
+	 *
+	 * @since 7.1.0
+	 */
+	protected function load_default_icons() {
+		if ( $this->default_icons_loaded ) {
+			return;
+		}
+
+		/*
+		 * Set before registering: _wp_register_default_icons() calls back into
+		 * this class, and this flag is what stops that from recursing.
+		 */
+		$this->default_icons_loaded = true;
+
+		_wp_register_default_icons();
+	}
+
 	/**
 	 * Utility method to retrieve the main instance of the class.
 	 *
diff --git a/src/wp-includes/default-filters.php b/src/wp-includes/default-filters.php
index 12ca0045b98b4..64d49f8cd9aa1 100644
--- a/src/wp-includes/default-filters.php
+++ b/src/wp-includes/default-filters.php
@@ -816,7 +816,10 @@
 
 // Icons.
 add_action( 'init', '_wp_register_default_icon_collections', 0 );
-add_action( 'init', '_wp_register_default_icons' );
+/*
+ * The default icons themselves are registered lazily, on first read of the
+ * icons registry. See WP_Icons_Registry::load_default_icons().
+ */
 
 // Add ignoredHookedBlocks metadata attribute to the template and template part post types.
 add_filter( 'rest_pre_insert_wp_template', 'inject_ignored_hooked_blocks_metadata_attributes' );
diff --git a/src/wp-includes/icons.php b/src/wp-includes/icons.php
index 0af4b6d61b873..144a29d531a16 100644
--- a/src/wp-includes/icons.php
+++ b/src/wp-includes/icons.php
@@ -98,6 +98,15 @@ function _wp_register_default_icon_collections() {
  * @access private
  */
 function _wp_register_default_icons() {
+	/*
+	 * Icons can be requested before `init` now that the default icons are
+	 * registered on first read, so make sure the collection they belong to
+	 * exists rather than relying on the `init` callback having run.
+	 */
+	if ( ! WP_Icon_Collections_Registry::get_instance()->is_registered( 'core' ) ) {
+		_wp_register_default_icon_collections();
+	}
+
 	$icons_directory = ABSPATH . WPINC . '/images/icon-library/';
 	$manifest_path   = ABSPATH . WPINC . '/assets/icon-library-manifest.php';
 
diff --git a/tests/phpunit/tests/icons/wpIconsRegistry.php b/tests/phpunit/tests/icons/wpIconsRegistry.php
index 42853d2666a95..a68d75a3a9715 100644
--- a/tests/phpunit/tests/icons/wpIconsRegistry.php
+++ b/tests/phpunit/tests/icons/wpIconsRegistry.php
@@ -453,4 +453,108 @@ public function test_get_content_returns_null_for_invalid_file( $contents, $exte
 
 		$this->assertNull( $icon['content'] );
 	}
+
+	/**
+	 * Resets the registry singleton so a test starts with the default icons
+	 * unloaded.
+	 *
+	 * Needed because tear_down() unregisters a test collection, and collection
+	 * unregistration reads the icon registry in order to drop that collection's
+	 * icons — which triggers the lazy load into the replacement instance.
+	 */
+	private function reset_registry() {
+		$reflection = new ReflectionClass( WP_Icons_Registry::class );
+		$property   = $reflection->getProperty( 'instance' );
+		if ( PHP_VERSION_ID < 80100 ) {
+			$property->setAccessible( true );
+		}
+		$property->setValue( null, null );
+
+		$this->registry = WP_Icons_Registry::get_instance();
+	}
+
+	/**
+	 * Reads the registered icons property directly, so that the assertion itself
+	 * does not trigger the lazy registration it is checking for.
+	 *
+	 * @return array Registered icons.
+	 */
+	private function get_registered_icons_property() {
+		$reflection = new ReflectionClass( WP_Icons_Registry::class );
+		$property   = $reflection->getProperty( 'registered_icons' );
+		if ( PHP_VERSION_ID < 80100 ) {
+			$property->setAccessible( true );
+		}
+
+		return $property->getValue( $this->registry );
+	}
+
+	/**
+	 * @ticket 00000
+	 *
+	 * @covers ::load_default_icons
+	 */
+	public function test_default_icons_are_not_registered_until_the_registry_is_read() {
+		$this->reset_registry();
+
+		$this->assertSame(
+			array(),
+			$this->get_registered_icons_property(),
+			'The default icons should not be registered before the registry is read.'
+		);
+	}
+
+	/**
+	 * @ticket 00000
+	 *
+	 * @covers ::load_default_icons
+	 */
+	public function test_default_icons_are_registered_on_first_read() {
+		$this->reset_registry();
+
+		$icon = $this->registry->get_registered_icon( 'core/arrow-left' );
+
+		$this->assertIsArray( $icon, 'A core icon should resolve without explicit registration.' );
+		$this->assertSame( 'core/arrow-left', $icon['name'] );
+		$this->assertStringStartsWith( '<svg', $icon['content'] );
+		$this->assertNotEmpty(
+			$this->get_registered_icons_property(),
+			'Reading the registry should have registered the default icons.'
+		);
+	}
+
+	/**
+	 * @ticket 00000
+	 *
+	 * @covers ::is_registered
+	 */
+	public function test_is_registered_triggers_default_icon_registration() {
+		$this->reset_registry();
+
+		$this->assertTrue( $this->registry->is_registered( 'core/arrow-left' ) );
+	}
+
+	/**
+	 * The default icons belong to the `core` collection, which is normally
+	 * registered on `init`. A lazy load can happen before that, so the collection
+	 * must be ensured at load time rather than assumed to exist.
+	 *
+	 * @ticket 00000
+	 *
+	 * @covers ::load_default_icons
+	 */
+	public function test_default_icons_load_even_if_core_collection_is_missing() {
+		$collections = WP_Icon_Collections_Registry::get_instance();
+		if ( $collections->is_registered( 'core' ) ) {
+			$collections->unregister( 'core' );
+		}
+		$this->assertFalse( $collections->is_registered( 'core' ) );
+
+		$this->reset_registry();
+
+		$icons = $this->registry->get_registered_icons();
+
+		$this->assertNotEmpty( $icons, 'Default icons should register even when the collection was not set up yet.' );
+		$this->assertTrue( $collections->is_registered( 'core' ), 'The core collection should be registered on demand.' );
+	}
 }