diff --git a/.changeset/fuzzy-shrimps-dream.md b/.changeset/fuzzy-shrimps-dream.md
new file mode 100644
index 000000000000..0ddab531acb5
--- /dev/null
+++ b/.changeset/fuzzy-shrimps-dream.md
@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: Add check for builtin custom elements in `set_custom_element_data`
diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index 2fa5d4541c48..8541da72f149 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -241,7 +241,7 @@ export function set_custom_element_data(node, prop, value) {
 			(setters_cache.has(node.nodeName) ||
 			// customElements may not be available in browser extension contexts
 			!customElements ||
-			customElements.get(node.tagName.toLowerCase())
+			customElements.get(node.getAttribute('is') || node.tagName.toLowerCase())
 				? get_setters(node).includes(prop)
 				: value && typeof value === 'object')
 		) {