diff --git a/extensions/amp-form/0.1/amp-form.js b/extensions/amp-form/0.1/amp-form.js
index b7a9c1f9a6a70..c7d289cccfb30 100644
--- a/extensions/amp-form/0.1/amp-form.js
+++ b/extensions/amp-form/0.1/amp-form.js
@@ -161,7 +161,7 @@ export class AmpForm {
     if (!this.shouldValidate_) {
       this.form_.setAttribute('amp-novalidate', '');
     }
-    this.form_.classList.add('-amp-form');
+    this.form_.classList.add('i-amphtml-form');
 
     const submitButtons = this.form_.querySelectorAll('[type="submit"]');
     /** @const @private {!Array<!Element>} */
@@ -721,7 +721,7 @@ function isDisabled_(element) {
 function installSubmissionHandlers(win) {
   onDocumentReady(win.document, doc => {
     toArray(doc.forms).forEach((form, index) => {
-      if (!form.classList.contains('-amp-form')) {
+      if (!form.classList.contains('i-amphtml-form')) {
         new AmpForm(form, `amp-form-${index}`);
       }
     });
diff --git a/extensions/amp-form/0.1/test/test-amp-form.js b/extensions/amp-form/0.1/test/test-amp-form.js
index ddf1d2bfb0516..811caf4dae543 100644
--- a/extensions/amp-form/0.1/test/test-amp-form.js
+++ b/extensions/amp-form/0.1/test/test-amp-form.js
@@ -147,7 +147,7 @@ describes.realWin('amp-form', {
     expect(form.addEventListener).to.be.calledWith('submit');
     expect(form.addEventListener).to.be.calledWith('blur');
     expect(form.addEventListener).to.be.calledWith('input');
-    expect(form.className).to.contain('-amp-form');
+    expect(form.className).to.contain('i-amphtml-form');
     document.body.removeChild(form);
   });
 
diff --git a/src/document-submit.js b/src/document-submit.js
index 99b5f06efb9d4..96a6ae090d88e 100644
--- a/src/document-submit.js
+++ b/src/document-submit.js
@@ -54,7 +54,7 @@ export function onDocumentFormSubmit_(e) {
 
   // amp-form extension will add novalidate to all forms to manually trigger
   // validation. In that case `novalidate` doesn't have the same meaning.
-  const isAmpFormMarked = form.classList.contains('-amp-form');
+  const isAmpFormMarked = form.classList.contains('i-amphtml-form');
   let shouldValidate;
   if (isAmpFormMarked) {
     shouldValidate = !form.hasAttribute('amp-novalidate');