diff --git a/builtins/amp-img.js b/builtins/amp-img.js
index 1593ab8ba730..8cc334756607 100644
--- a/builtins/amp-img.js
+++ b/builtins/amp-img.js
@@ -27,7 +27,8 @@ import {registerElement} from '../src/service/custom-element-registry';
 const TAG = 'amp-img';
 
 /**
- * Attributes to propagate to internal image when changed externally.
+ * Attributes to propagate to internal image when changed externally. The
+ * `sizes` attribute is handled separately to prioritize `img-sizes`.
  * @type {!Array<string>}
  */
 const ATTRIBUTES_TO_PROPAGATE = [
@@ -39,7 +40,6 @@ const ATTRIBUTES_TO_PROPAGATE = [
   'aria-labelledby',
   'srcset',
   'src',
-  'sizes',
 ];
 
 export class AmpImg extends BaseElement {
@@ -69,6 +69,30 @@ export class AmpImg extends BaseElement {
     this.sizesWidth_ = 0;
   }
 
+  /**
+   * @return {?string} The sizes value to use for the img, preferring
+   *    `img-sizes` and falling back to `sizes`.
+   */
+  getSizesValueToUse_() {
+    return (
+      this.element.getAttribute('img-sizes') ||
+      this.element.getAttribute('sizes')
+    );
+  }
+
+  /**
+   * If present, propagates the `img-sizes` or `sizes` value to the internal
+   * img.
+   */
+  propagateSizes() {
+    const sizes = this.getSizesValueToUse_();
+    if (sizes === null) {
+      this.img_.removeAttribute('sizes');
+    } else {
+      this.img_.setAttribute('sizes', sizes);
+    }
+  }
+
   /** @override */
   mutatedAttributesCallback(mutations) {
     if (this.img_) {
@@ -97,6 +121,9 @@ export class AmpImg extends BaseElement {
         this.img_,
         /* opt_removeMissingAttrs */ true
       );
+      if (mutations['sizes']) {
+        this.propagateSizes();
+      }
       guaranteeSrcForSrcsetUnsupportedBrowsers(this.img_);
     }
   }
@@ -176,6 +203,7 @@ export class AmpImg extends BaseElement {
     }
 
     this.propagateAttributes(ATTRIBUTES_TO_PROPAGATE, this.img_);
+    this.propagateSizes();
     guaranteeSrcForSrcsetUnsupportedBrowsers(this.img_);
     this.maybeGenerateSizes_();
     this.applyFillContent(this.img_, true);
@@ -194,7 +222,7 @@ export class AmpImg extends BaseElement {
       return;
     }
     // No need to generate sizes if already present.
-    const sizes = this.element.getAttribute('sizes');
+    const sizes = this.getSizesValueToUse_();
     if (sizes) {
       return;
     }
diff --git a/builtins/amp-img.md b/builtins/amp-img.md
index fa0b36b6315d..ef0676e79fa2 100644
--- a/builtins/amp-img.md
+++ b/builtins/amp-img.md
@@ -125,6 +125,10 @@ For the `<img>` tag in `HTML`, the `sizes` attribute is used in conjunction with
 
 See [Responsive images with srcset, sizes & heights](https://amp.dev/documentation/guides-and-tutorials/develop/style_and_layout/art_direction) for usage of `sizes` and `srcset`.
 
+**img-sizes**
+
+Like `sizes` above, sets the [`sizes`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img#attr-sizes) attribute on the underlying `img`. Unlike `sizes`, this does not change the width of the `amp-img` and is only used by the browser for picking which source to use from the `srcset`.
+
 **alt**
 
 A string of alternate text, similar to the `alt` attribute on `img`.
diff --git a/test/unit/test-amp-img.js b/test/unit/test-amp-img.js
index 0c25f28225f3..0958ae107230 100644
--- a/test/unit/test-amp-img.js
+++ b/test/unit/test-amp-img.js
@@ -188,6 +188,67 @@ describe('amp-img', () => {
     });
   });
 
+  it('should propagate srcset and img-sizes', async () => {
+    const ampImg = await getImg({
+      src: '/examples/img/sample.jpg',
+      srcset: SRCSET_STRING,
+      'img-sizes': '(max-width: 320px) 640px, 100vw',
+      width: 320,
+      height: 240,
+    });
+    const img = ampImg.querySelector('img');
+    expect(img.getAttribute('srcset')).to.equal(SRCSET_STRING);
+    expect(img.getAttribute('sizes')).to.equal(
+      '(max-width: 320px) 640px, 100vw'
+    );
+  });
+
+  it('should prefer img-sizes to sizes', async () => {
+    const ampImg = await getImg({
+      src: '/examples/img/sample.jpg',
+      srcset: SRCSET_STRING,
+      sizes: '(max-width: 320px) 640px, 100vw',
+      'img-sizes': '640px',
+      width: 320,
+      height: 240,
+    });
+    const img = ampImg.querySelector('img');
+    expect(img.getAttribute('srcset')).to.equal(SRCSET_STRING);
+    expect(img.getAttribute('sizes')).to.equal('640px');
+  });
+
+  it('should update sizes on mutation', async () => {
+    const ampImg = await getImg({
+      src: '/examples/img/sample.jpg',
+      srcset: SRCSET_STRING,
+      sizes: '640px',
+      width: 300,
+      height: 200,
+    });
+    const impl = ampImg.implementation_;
+
+    ampImg.setAttribute('sizes', '50vw');
+    impl.mutatedAttributesCallback({sizes: '50vw'});
+
+    expect(impl.img_.getAttribute('sizes')).to.equal('50vw');
+  });
+
+  it('should ignore sizes mutations when img-sizes is used', async () => {
+    const ampImg = await getImg({
+      src: '/examples/img/sample.jpg',
+      srcset: SRCSET_STRING,
+      'img-sizes': '640px',
+      width: 300,
+      height: 200,
+    });
+    const impl = ampImg.implementation_;
+
+    ampImg.setAttribute('sizes', '50vw');
+    impl.mutatedAttributesCallback({sizes: '50vw'});
+
+    expect(impl.img_.getAttribute('sizes')).to.equal('640px');
+  });
+
   describe('#fallback on initial load', () => {
     let el;
     let impl;
@@ -530,6 +591,23 @@ describe('amp-img', () => {
         });
     });
 
+    it('should not generate sizes for amp-imgs that already have img-sizes', async () => {
+      const ampImg = await getImg({
+        src: '/examples/img/sample.jpg',
+        srcset: SRCSET_STRING,
+        'img-sizes': '50vw',
+        width: 300,
+        height: 200,
+      });
+
+      const impl = ampImg.implementation_;
+      impl.buildCallback();
+      await impl.layoutCallback();
+
+      const img = impl.img_;
+      expect(img.getAttribute('sizes')).to.equal('50vw');
+    });
+
     it('should not generate sizes for amp-imgs with layout intrinsic', () => {
       let impl;
       return getImg({
diff --git a/validator/validator-main.protoascii b/validator/validator-main.protoascii
index 347fce175197..2d526f298df8 100644
--- a/validator/validator-main.protoascii
+++ b/validator/validator-main.protoascii
@@ -5210,6 +5210,8 @@ tags: {  # <amp-img>
   attrs: { name: "object-fit" }
   attrs: { name: "object-position" }
   attrs: { name: "placeholder" }
+  # Used to set sizes on the <img> without AMP's sizes behavior
+  attrs: { name: "img-sizes" }
   # <amp-bind>
   attrs: { name: "[alt]" }
   attrs: { name: "[attribution]" }