diff --git a/documentation/docs/98-reference/.generated/compile-errors.md b/documentation/docs/98-reference/.generated/compile-errors.md
index c5703c636b..083b081001 100644
--- a/documentation/docs/98-reference/.generated/compile-errors.md
+++ b/documentation/docs/98-reference/.generated/compile-errors.md
@@ -1102,9 +1102,11 @@ Value must be %list%, if specified
 ### svelte_options_invalid_customelement_shadow
 
 ```
-"shadow" must be either "open" or "none"
+"shadow" must be either "open", "none" or `ShadowRootInit`
 ```
 
+See https://developer.mozilla.org/en-US/docs/Web/API/Element/attachShadow#options for more information on valid shadow root constructor options
+
 ### svelte_options_invalid_tagname
 
 ```
diff --git a/packages/svelte/elements.d.ts b/packages/svelte/elements.d.ts
index 17ff100729..34ed251400 100644
--- a/packages/svelte/elements.d.ts
+++ b/packages/svelte/elements.d.ts
@@ -2061,7 +2061,7 @@ export interface SvelteHTMLElements {
 			| undefined
 			| {
 					tag?: string;
-					shadow?: 'open' | 'none' | undefined;
+					shadow?: 'open' | 'none' | ShadowRootInit | undefined;
 					props?:
 						| Record<
 								string,
diff --git a/packages/svelte/messages/compile-errors/template.md b/packages/svelte/messages/compile-errors/template.md
index ac95bfe4a7..14c9be52e2 100644
--- a/packages/svelte/messages/compile-errors/template.md
+++ b/packages/svelte/messages/compile-errors/template.md
@@ -411,7 +411,9 @@ HTML restricts where certain elements can appear. In case of a violation the bro
 
 ## svelte_options_invalid_customelement_shadow
 
-> "shadow" must be either "open" or "none"
+> "shadow" must be either "open", "none" or `ShadowRootInit`
+
+See https://developer.mozilla.org/en-US/docs/Web/API/Element/attachShadow#options for more information on valid shadow root constructor options
 
 ## svelte_options_invalid_tagname
 
diff --git a/packages/svelte/src/compiler/errors.js b/packages/svelte/src/compiler/errors.js
index 5e3968215f..0cc51e39a3 100644
--- a/packages/svelte/src/compiler/errors.js
+++ b/packages/svelte/src/compiler/errors.js
@@ -1550,12 +1550,12 @@ export function svelte_options_invalid_customelement_props(node) {
 }
 
 /**
- * "shadow" must be either "open" or "none"
+ * "shadow" must be either "open", "none" or `ShadowRootInit`
  * @param {null | number | NodeLike} node
  * @returns {never}
  */
 export function svelte_options_invalid_customelement_shadow(node) {
-	e(node, 'svelte_options_invalid_customelement_shadow', `"shadow" must be either "open" or "none"\nhttps://svelte.dev/e/svelte_options_invalid_customelement_shadow`);
+	e(node, 'svelte_options_invalid_customelement_shadow', `"shadow" must be either "open", "none" or \`ShadowRootInit\`\nhttps://svelte.dev/e/svelte_options_invalid_customelement_shadow`);
 }
 
 /**
diff --git a/packages/svelte/src/compiler/phases/1-parse/read/options.js b/packages/svelte/src/compiler/phases/1-parse/read/options.js
index a36e101468..24b5f6967b 100644
--- a/packages/svelte/src/compiler/phases/1-parse/read/options.js
+++ b/packages/svelte/src/compiler/phases/1-parse/read/options.js
@@ -133,11 +133,49 @@ export default function read_options(node) {
 
 				const shadow = properties.find(([name]) => name === 'shadow')?.[1];
 				if (shadow) {
-					const shadowdom = shadow?.value;
-					if (shadowdom !== 'open' && shadowdom !== 'none') {
-						e.svelte_options_invalid_customelement_shadow(shadow);
+					if (shadow.type === 'Literal') {
+						if (shadow.value !== 'open' && shadow.value !== 'none') {
+							e.svelte_options_invalid_customelement_shadow(attribute);
+						}
+						ce.shadow = shadow.value;
+					} else if (shadow.type === 'ObjectExpression') {
+						ce.shadow = { mode: 'open' };
+						for (const property of /** @type {ObjectExpression} */ (shadow).properties) {
+							if (
+								property.type !== 'Property' ||
+								property.computed ||
+								property.key.type !== 'Identifier' ||
+								property.value.type !== 'Literal'
+							) {
+								e.svelte_options_invalid_customelement_shadow(attribute);
+							}
+
+							if (property.key.name === 'mode') {
+								if (!['open', 'closed'].includes(/** @type {string} */ (property.value.value))) {
+									e.svelte_options_invalid_customelement_shadow(attribute);
+								}
+								ce.shadow.mode = /** @type {any} */ (property.value.value);
+							} else if (property.key.name === 'slotAssignment') {
+								if (!['named', 'manual'].includes(/** @type {string} */ (property.value.value))) {
+									e.svelte_options_invalid_customelement_shadow(attribute);
+								}
+								ce.shadow.slotAssignment = /** @type {any} */ (property.value.value);
+							} else if (
+								property.key.name === 'clonable' ||
+								property.key.name === 'delegatesFocus' ||
+								property.key.name === 'serializable'
+							) {
+								if (typeof property.value.value !== 'boolean') {
+									e.svelte_options_invalid_customelement_shadow(attribute);
+								}
+								ce.shadow[property.key.name] = property.value.value;
+							} else {
+								e.svelte_options_invalid_customelement_shadow(attribute);
+							}
+						}
+					} else {
+						e.svelte_options_invalid_customelement_shadow(attribute);
 					}
-					ce.shadow = shadowdom;
 				}
 
 				const extend = properties.find(([name]) => name === 'extend')?.[1];
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
index f51042eb7c..664d980149 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
@@ -643,7 +643,24 @@ export function client_component(analysis, options) {
 		const accessors_str = b.array(
 			analysis.exports.map(({ name, alias }) => b.literal(alias ?? name))
 		);
-		const use_shadow_dom = typeof ce === 'boolean' || ce.shadow !== 'none' ? true : false;
+
+		/** @type {ShadowRootInit | {}} */
+		let ce_shadow_root_init = {};
+		if (typeof ce === 'boolean' || ce.shadow === 'open' || ce.shadow === undefined) {
+			ce_shadow_root_init = { mode: 'open' };
+		} else if (ce.shadow === 'none') {
+			ce_shadow_root_init = {};
+		} else if (typeof ce.shadow === 'object') {
+			ce_shadow_root_init = ce.shadow;
+		}
+
+		/** @type {ESTree.Property[]} */
+		const shadow_root_init_str = Object.entries(ce_shadow_root_init).map(([key, value]) =>
+			b.init(key, b.literal(value))
+		);
+		const shadow_root_init = shadow_root_init_str.length
+			? b.object(shadow_root_init_str)
+			: undefined;
 
 		const create_ce = b.call(
 			'$.create_custom_element',
@@ -651,7 +668,7 @@ export function client_component(analysis, options) {
 			b.object(props_str),
 			slots_str,
 			accessors_str,
-			b.literal(use_shadow_dom),
+			shadow_root_init,
 			/** @type {any} */ (typeof ce !== 'boolean' ? ce.extend : undefined)
 		);
 
diff --git a/packages/svelte/src/compiler/types/template.d.ts b/packages/svelte/src/compiler/types/template.d.ts
index 9cf498d187..039c508727 100644
--- a/packages/svelte/src/compiler/types/template.d.ts
+++ b/packages/svelte/src/compiler/types/template.d.ts
@@ -88,7 +88,7 @@ export namespace AST {
 		css?: 'injected';
 		customElement?: {
 			tag?: string;
-			shadow?: 'open' | 'none';
+			shadow?: 'open' | 'none' | (ShadowRootInit & { clonable?: boolean });
 			props?: Record<
 				string,
 				{
diff --git a/packages/svelte/src/internal/client/dom/elements/custom-element.js b/packages/svelte/src/internal/client/dom/elements/custom-element.js
index 2d118bfab3..d4e550291b 100644
--- a/packages/svelte/src/internal/client/dom/elements/custom-element.js
+++ b/packages/svelte/src/internal/client/dom/elements/custom-element.js
@@ -35,18 +35,22 @@ if (typeof HTMLElement === 'function') {
 		$$l_u = new Map();
 		/** @type {any} The managed render effect for reflecting attributes */
 		$$me;
+		/** @type {ShadowRoot | null} The ShadowRoot of the custom element */
+		$$shadowRoot = null;
 
 		/**
 		 * @param {*} $$componentCtor
 		 * @param {*} $$slots
-		 * @param {*} use_shadow_dom
+		 * @param {ShadowRootInit | undefined} shadow_root_init
 		 */
-		constructor($$componentCtor, $$slots, use_shadow_dom) {
+		constructor($$componentCtor, $$slots, shadow_root_init) {
 			super();
 			this.$$ctor = $$componentCtor;
 			this.$$s = $$slots;
-			if (use_shadow_dom) {
-				this.attachShadow({ mode: 'open' });
+			if (shadow_root_init) {
+				// We need to store the reference to shadow root, because `closed` shadow root cannot be
+				// accessed with `this.shadowRoot`.
+				this.$$shadowRoot = this.attachShadow(shadow_root_init);
 			}
 		}
 
@@ -136,7 +140,7 @@ if (typeof HTMLElement === 'function') {
 				}
 				this.$$c = createClassComponent({
 					component: this.$$ctor,
-					target: this.shadowRoot || this,
+					target: this.$$shadowRoot || this,
 					props: {
 						...this.$$d,
 						$$slots,
@@ -277,7 +281,7 @@ function get_custom_elements_slots(element) {
  * @param {Record<string, CustomElementPropDefinition>} props_definition  The props to observe
  * @param {string[]} slots  The slots to create
  * @param {string[]} exports  Explicitly exported values, other than props
- * @param {boolean} use_shadow_dom  Whether to use shadow DOM
+ * @param {ShadowRootInit | undefined} shadow_root_init  Options passed to shadow DOM constructor
  * @param {(ce: new () => HTMLElement) => new () => HTMLElement} [extend]
  */
 export function create_custom_element(
@@ -285,12 +289,12 @@ export function create_custom_element(
 	props_definition,
 	slots,
 	exports,
-	use_shadow_dom,
+	shadow_root_init,
 	extend
 ) {
 	let Class = class extends SvelteElement {
 		constructor() {
-			super(Component, slots, use_shadow_dom);
+			super(Component, slots, shadow_root_init);
 			this.$$p_d = props_definition;
 		}
 		static get observedAttributes() {
diff --git a/packages/svelte/types/index.d.ts b/packages/svelte/types/index.d.ts
index 5e3ca77eb5..350ebcd1cb 100644
--- a/packages/svelte/types/index.d.ts
+++ b/packages/svelte/types/index.d.ts
@@ -1224,7 +1224,7 @@ declare module 'svelte/compiler' {
 			css?: 'injected';
 			customElement?: {
 				tag?: string;
-				shadow?: 'open' | 'none';
+				shadow?: 'open' | 'none' | (ShadowRootInit & { clonable?: boolean });
 				props?: Record<
 					string,
 					{