diff --git a/.changeset/fix-static-renderer-namespaced-tag.md b/.changeset/fix-static-renderer-namespaced-tag.md
new file mode 100644
index 0000000000..341c4814cc
--- /dev/null
+++ b/.changeset/fix-static-renderer-namespaced-tag.md
@@ -0,0 +1,5 @@
+---
+'@tiptap/static-renderer': patch
+---
+
+`renderToHTMLString` now closes namespaced elements correctly. The closing tag no longer repeats the `xmlns` declaration, and tags that cannot self-close, such as `div` or `iframe`, are no longer written as self-closing.
diff --git a/packages/static-renderer/__tests__/json-string.spec.ts b/packages/static-renderer/__tests__/json-string.spec.ts
index e8e99993b9..53bcad42b8 100644
--- a/packages/static-renderer/__tests__/json-string.spec.ts
+++ b/packages/static-renderer/__tests__/json-string.spec.ts
@@ -1,4 +1,4 @@
-import { extensions as coreExtensions } from '@tiptap/core'
+import { extensions as coreExtensions, Node as CoreNode } from '@tiptap/core'
 import Bold from '@tiptap/extension-bold'
 import CodeBlock from '@tiptap/extension-code-block'
 import Document from '@tiptap/extension-document'
@@ -562,3 +562,37 @@ describe('static render json to string (with prosemirror)', () => {
     expect(html).toMatch(/data-toc-id="[^"]+"/)
   })
 })
+
+describe('namespaced DOMOutputSpec', () => {
+  // ProseMirror encodes a namespaced spec as `"<namespace> <localName>"`.
+  const Diagram = CoreNode.create({
+    name: 'diagram',
+    group: 'block',
+    content: 'text*',
+    renderHTML: () => ['http://www.w3.org/2000/svg svg', { width: '100' }, 0],
+  })
+
+  it('keeps the xmlns attribute out of the closing tag', () => {
+    const html = renderToHTMLString({
+      content: {
+        type: 'doc',
+        content: [{ type: 'diagram', content: [{ type: 'text', text: 'hi' }] }],
+      },
+      extensions: [Document, Paragraph, Text, Diagram],
+    })
+
+    // The namespace belongs on the opening tag only.
+    expect(html).toContain('<svg xmlns="http://www.w3.org/2000/svg"')
+    expect(html).toMatch(/<\/svg>$/)
+  })
+
+  it('still resolves non-self-closing tags by their local name', () => {
+    // `div` is in NON_SELF_CLOSING_TAGS, so the namespaced spec must not self-close.
+    const html = domOutputSpecToHTMLString([
+      'http://www.w3.org/1999/xhtml div',
+      { class: 'wrapper' },
+    ])()
+
+    expect(html).toContain('></div')
+  })
+})
diff --git a/packages/static-renderer/src/pm/html-string/html-string.ts b/packages/static-renderer/src/pm/html-string/html-string.ts
index 2902faef39..5d31bfb347 100644
--- a/packages/static-renderer/src/pm/html-string/html-string.ts
+++ b/packages/static-renderer/src/pm/html-string/html-string.ts
@@ -46,47 +46,47 @@ export function domOutputSpecToHTMLString(
   }
   if (typeof content === 'object' && 'length' in content) {
     const [_tag, attrs, children, ...rest] = content as DOMOutputSpecArray
-    let tag = _tag
-    const parts = tag.split(' ')
-
-    if (parts.length > 1) {
-      tag = `${parts[1]} xmlns="${parts[0]}"`
-    }
+    // ProseMirror encodes a namespaced spec as `"<namespace> <localName>"`.
+    // Only the opening tag carries the `xmlns` attribute: the closing tag and the
+    // `NON_SELF_CLOSING_TAGS` lookup below both need the bare local name.
+    const parts = _tag.split(' ')
+    const tag = parts.length > 1 ? parts[1] : _tag
+    const openTag = parts.length > 1 ? `${parts[1]} xmlns="${parts[0]}"` : _tag
 
     if (attrs === undefined) {
-      return () => `<${tag}/>`
+      return () => `<${openTag}/>`
     }
     if (attrs === 0) {
-      return child => `<${tag}>${serializeChildrenToHTMLString(child)}</${tag}>`
+      return child => `<${openTag}>${serializeChildrenToHTMLString(child)}</${tag}>`
     }
     if (typeof attrs === 'object') {
       if (Array.isArray(attrs)) {
         if (children === undefined) {
           return child =>
-            `<${tag}>${domOutputSpecToHTMLString(attrs as DOMOutputSpecArray)(child)}</${tag}>`
+            `<${openTag}>${domOutputSpecToHTMLString(attrs as DOMOutputSpecArray)(child)}</${tag}>`
         }
         if (children === 0) {
           return child =>
-            `<${tag}>${domOutputSpecToHTMLString(attrs as DOMOutputSpecArray)(child)}</${tag}>`
+            `<${openTag}>${domOutputSpecToHTMLString(attrs as DOMOutputSpecArray)(child)}</${tag}>`
         }
         return child =>
-          `<${tag}>${domOutputSpecToHTMLString(attrs as DOMOutputSpecArray)(child)}${[children]
+          `<${openTag}>${domOutputSpecToHTMLString(attrs as DOMOutputSpecArray)(child)}${[children]
             .concat(rest)
             .map(a => domOutputSpecToHTMLString(a)(child))}</${tag}>`
       }
       if (children === undefined) {
         if (NON_SELF_CLOSING_TAGS.has(tag)) {
-          return () => `<${tag}${serializeAttrsToHTMLString(attrs)}></${tag}>`
+          return () => `<${openTag}${serializeAttrsToHTMLString(attrs)}></${tag}>`
         }
-        return () => `<${tag}${serializeAttrsToHTMLString(attrs)}/>`
+        return () => `<${openTag}${serializeAttrsToHTMLString(attrs)}/>`
       }
       if (children === 0) {
         return child =>
-          `<${tag}${serializeAttrsToHTMLString(attrs)}>${serializeChildrenToHTMLString(child)}</${tag}>`
+          `<${openTag}${serializeAttrsToHTMLString(attrs)}>${serializeChildrenToHTMLString(child)}</${tag}>`
       }
 
       return child =>
-        `<${tag}${serializeAttrsToHTMLString(attrs)}>${[children]
+        `<${openTag}${serializeAttrsToHTMLString(attrs)}>${[children]
           .concat(rest)
           .map(a => domOutputSpecToHTMLString(a)(child))
           .join('')}</${tag}>`