diff --git a/packages/svelte2tsx/src/htmlxtojsx_v2/nodes/Attribute.ts b/packages/svelte2tsx/src/htmlxtojsx_v2/nodes/Attribute.ts
index 8a4221b88..3f44c6a05 100644
--- a/packages/svelte2tsx/src/htmlxtojsx_v2/nodes/Attribute.ts
+++ b/packages/svelte2tsx/src/htmlxtojsx_v2/nodes/Attribute.ts
@@ -113,7 +113,7 @@ export function handleAttribute(
         if (
             !preserveCase &&
             !svgAttributes.find((x) => x == name) &&
-            !(element instanceof Element && element.tagName.includes('-')) &&
+            !(element instanceof Element && element.isCustomElement()) &&
             !(svelte5Plus && name.startsWith('on'))
         ) {
             return name.toLowerCase();
diff --git a/packages/svelte2tsx/src/htmlxtojsx_v2/nodes/Element.ts b/packages/svelte2tsx/src/htmlxtojsx_v2/nodes/Element.ts
index bd8947c64..84ba4bb9b 100644
--- a/packages/svelte2tsx/src/htmlxtojsx_v2/nodes/Element.ts
+++ b/packages/svelte2tsx/src/htmlxtojsx_v2/nodes/Element.ts
@@ -255,6 +255,20 @@ export class Element {
         }
     }
 
+    isCustomElement(): boolean {
+        if (this.tagName.includes('-')) {
+            return true;
+        }
+        if (
+            this.node.attributes
+                ?.find((a: BaseNode) => a.name === 'is')
+                ?.value[0]?.data.includes('-')
+        ) {
+            return true;
+        }
+        return false;
+    }
+
     private getStartTransformation(): TransformationArray {
         const createElement = `${this.typingsNamespace}.createElement`;
         const addActions = () => {
diff --git a/packages/svelte2tsx/test/htmlx2jsx/samples/custom-element-builtin-attribute/expectedv2.js b/packages/svelte2tsx/test/htmlx2jsx/samples/custom-element-builtin-attribute/expectedv2.js
new file mode 100644
index 000000000..615ef7551
--- /dev/null
+++ b/packages/svelte2tsx/test/htmlx2jsx/samples/custom-element-builtin-attribute/expectedv2.js
@@ -0,0 +1 @@
+ { svelteHTML.createElement("div", {          "is":`custom-element`,"camelCase":`true`,"kebab-case":`true`,"PascalCase":`true`,"snake_case":`true`,});}
diff --git a/packages/svelte2tsx/test/htmlx2jsx/samples/custom-element-builtin-attribute/input.svelte b/packages/svelte2tsx/test/htmlx2jsx/samples/custom-element-builtin-attribute/input.svelte
new file mode 100644
index 000000000..f5f7b1b8d
--- /dev/null
+++ b/packages/svelte2tsx/test/htmlx2jsx/samples/custom-element-builtin-attribute/input.svelte
@@ -0,0 +1 @@
+<div is="custom-element" camelCase="true" kebab-case="true" PascalCase="true" snake_case="true" />