diff --git a/build-system/tasks/presubmit-checks.js b/build-system/tasks/presubmit-checks.js
index 573aee710cc64..36d7f83addc04 100644
--- a/build-system/tasks/presubmit-checks.js
+++ b/build-system/tasks/presubmit-checks.js
@@ -482,15 +482,17 @@ const forbiddenTerms = {
       // https://amp.dev/documentation/guides-and-tutorials/learn/a4a_spec
       'src/services.js',
       'src/service/cid-impl.js',
-      'extensions/amp-user-notification/0.1/amp-user-notification.js',
+      'extensions/amp-ad-network-adsense-impl/0.1/amp-ad-network-adsense-impl.js',
       'extensions/amp-app-banner/0.1/amp-app-banner.js',
       'extensions/amp-consent/0.1/consent-state-manager.js',
+      'extensions/amp-user-notification/0.1/amp-user-notification.js',
     ],
   },
   'localStorage': {
     message: requiresReviewPrivacy,
     whitelist: [
       'extensions/amp-access/0.1/amp-access-iframe.js',
+      'extensions/amp-ad-network-adsense-impl/0.1/amp-ad-network-adsense-impl.js',
       'extensions/amp-script/0.1/amp-script.js',
       'extensions/amp-web-push/0.1/amp-web-push-helper-frame.js',
       'extensions/amp-web-push/0.1/amp-web-push-permission-dialog.js',
diff --git a/extensions/amp-a4a/0.1/amp-a4a.js b/extensions/amp-a4a/0.1/amp-a4a.js
index 3316a27bff6cc..a423f2d92dcbc 100644
--- a/extensions/amp-a4a/0.1/amp-a4a.js
+++ b/extensions/amp-a4a/0.1/amp-a4a.js
@@ -372,7 +372,9 @@ export class AmpA4A extends AMP.BaseElement {
     return this.isRelayoutNeededFlag;
   }
 
-  /** @override */
+  /** @override
+      @return {!Promise|undefined}
+  */
   buildCallback() {
     this.creativeSize_ = {
       width: this.element.getAttribute('width'),
diff --git a/extensions/amp-ad-network-adsense-impl/0.1/amp-ad-network-adsense-impl.js b/extensions/amp-ad-network-adsense-impl/0.1/amp-ad-network-adsense-impl.js
index 6bd8ac07a0d1b..3e92105ba5052 100644
--- a/extensions/amp-ad-network-adsense-impl/0.1/amp-ad-network-adsense-impl.js
+++ b/extensions/amp-ad-network-adsense-impl/0.1/amp-ad-network-adsense-impl.js
@@ -76,6 +76,15 @@ const ADSENSE_BASE_URL = 'https://googleads.g.doubleclick.net/pagead/ads';
 /** @const {string} */
 const TAG = 'amp-ad-network-adsense-impl';
 
+/** @const {!{branch: string, control: string, experiment: string}}
+    @visibleForTesting
+*/
+export const AD_SIZE_OPTIMIZATION_EXP = {
+  branch: 'adsense-ad-size-optimization',
+  control: '368226510',
+  experiment: '368226511',
+};
+
 /**
  * Shared state for AdSense ad slots. This is used primarily for ad request url
  * parameters that depend on previous slots.
@@ -161,6 +170,12 @@ export class AmpAdNetworkAdsenseImpl extends AmpA4A {
      * @private {boolean}
      */
     this.shouldSandbox_ = false;
+
+    /**
+     * Testing callback invoked when auto ad size settings are persisted in
+     * localstorage.
+     * @private {function()} */
+    this.onAutoAdSizeSettingsStored_ = () => {};
   }
 
   /**
@@ -238,7 +253,9 @@ export class AmpAdNetworkAdsenseImpl extends AmpA4A {
     return getAmpAdRenderOutsideViewport(this.element) || 3;
   }
 
-  /** @override */
+  /** @override
+      @return {!Promise|undefined}.
+  */
   buildCallback() {
     super.buildCallback();
     this.identityTokenPromise_ = Services.viewerForDoc(this.getAmpDoc())
@@ -246,26 +263,148 @@ export class AmpAdNetworkAdsenseImpl extends AmpA4A {
       .then(() =>
         getIdentityToken(this.win, this.getAmpDoc(), super.getConsentPolicy())
       );
-    this.autoFormat_ = this.element.getAttribute('data-auto-format') || '';
 
-    if (this.isResponsive_()) {
-      // Attempt to resize to the correct height. The width should already be
-      // 100vw, but is fixed here so that future resizes of the viewport don't
-      // affect it.
-      const viewportSize = this.getViewport().getSize();
-      return this.attemptChangeSize(
-        AmpAdNetworkAdsenseImpl.getResponsiveHeightForContext_(
-          this.autoFormat_,
-          viewportSize,
-          this.element
-        ),
-        viewportSize.width
-      ).catch(() => {});
+    return this.maybeUpgradeToResponsive().then(() => {
+      this.autoFormat_ = this.element.getAttribute('data-auto-format') || '';
+
+      if (this.isResponsive_()) {
+        // Attempt to resize to the correct height. The width should already be
+        // 100vw, but is fixed here so that future resizes of the viewport don't
+        // affect it.
+        const viewportSize = this.getViewport().getSize();
+        return this.attemptChangeSize(
+          AmpAdNetworkAdsenseImpl.getResponsiveHeightForContext_(
+            this.autoFormat_,
+            viewportSize,
+            this.element
+          ),
+          viewportSize.width
+        ).catch(() => {});
+      }
+
+      // This should happen last, as some diversion criteria rely on some of the
+      // preceding logic (specifically responsive logic).
+      this.divertExperiments();
+      this.maybeAddSinglePassExperiment();
+    });
+  }
+
+  /**
+   * Upgrades the ad unit to responsive if there is an opt-in setting in localstorage.
+   * See https://github.com/ampproject/amphtml/issues/23568 for design.
+   * @return {!Promise} a promise that resolves when any upgrade is complete.
+   */
+  maybeUpgradeToResponsive() {
+    if (!this.isInAdSizeOptimizationExperimentBranch()) {
+      return Promise.resolve();
+    }
+    // If the ad unit is already responsive, there's nothing to do.
+    // Note that we can't use this.autoFormat_ here because it's not
+    // populated yet.
+    if (this.element.hasAttribute('data-auto-format')) {
+      return Promise.resolve();
     }
-    // This should happen last, as some diversion criteria rely on some of the
-    // preceding logic (specifically responsive logic).
-    this.divertExperiments();
-    this.maybeAddSinglePassExperiment();
+    return (
+      Services.storageForDoc(this.element)
+        .then(storage => storage.get(this.getAdSizeOptimizationStorageKey()))
+        .then(isAdSizeOptimizationEnabled => {
+          if (isAdSizeOptimizationEnabled) {
+            this.upgradeToResponsive();
+          }
+        })
+        // Do nothing if we fail to read localstorage.
+        .catch(() => {
+          dev().warn(TAG, 'Failed to look up publisher ad size settings.');
+        })
+    );
+  }
+
+  /**
+   * Actually upgrades the ad unit to responsive.
+   */
+  upgradeToResponsive() {
+    this.element.setAttribute('height', ADSENSE_RSPV_WHITELISTED_HEIGHT);
+    this.element.setAttribute('width', '100vw');
+    this.element.setAttribute('data-full-width', '');
+    this.element.setAttribute('data-auto-format', 'rspv');
+
+    devAssert(this.isValidElement(), 'Upgraded element is not valid.');
+  }
+
+  /**
+   * Sets up a listener for a pingback of publisher settings in the ad response and
+   * writing such settings to localstorage.
+   *
+   * Note that we can have multiple listeners on the same page, which is okay because
+   * the settings for publishers should not change between different slots.
+   *
+   * See https://github.com/ampproject/amphtml/issues/23568 for design.
+   */
+  attachSettingsListener() {
+    const listener = event => {
+      if (event['source'] != this.iframe.contentWindow) {
+        return;
+      }
+      const data = getData(event);
+      // data will look like this:
+      // {
+      //   'googMsgType': 'adsense-settings',
+      //   'adClient': 'ca-pub-123',
+      //   'enableAutoAdSize': '1'
+      // }
+      if (!!data && data['googMsgType'] != 'adsense-settings') {
+        return;
+      }
+      if (data['adClient'] != this.getAdClientId()) {
+        return;
+      }
+      const autoAdSizeStatus = data['enableAutoAdSize'] == '1';
+      this.win.removeEventListener('message', listener);
+      Services.storageForDoc(this.element)
+        .then(storage =>
+          storage
+            .set(this.getAdSizeOptimizationStorageKey(), autoAdSizeStatus)
+            .then(() => {
+              dev().info(
+                TAG,
+                `Saved publisher auto ad size setting: ${autoAdSizeStatus}`
+              );
+              this.onAutoAdSizeSettingsStored_();
+            })
+        )
+        // Do nothing if we fail to write to localstorage.
+        .catch(() => {
+          dev().warn(TAG, 'Failed to persist publisher auto ad size setting.');
+        });
+    };
+    this.win.addEventListener('message', listener);
+  }
+
+  /**
+   * Selects into the ad size optimization experiment.
+   * Note that this needs to be done before responsive sizing, so it must
+   * be separate from divertExperiments below.
+   * @return {boolean}
+   */
+  isInAdSizeOptimizationExperimentBranch() {
+    const experimentInfoMap = /** @type {!Object<string,
+        !../../../src/experiments.ExperimentInfo>} */ ({
+      [[AD_SIZE_OPTIMIZATION_EXP.branch]]: {
+        isTrafficEligible: () => this.isResponsive_(),
+        branches: [
+          [AD_SIZE_OPTIMIZATION_EXP.control],
+          [AD_SIZE_OPTIMIZATION_EXP.experiment],
+        ],
+      },
+    });
+    const setExps = randomlySelectUnsetExperiments(this.win, experimentInfoMap);
+    Object.keys(setExps).forEach(expName =>
+      addExperimentIdToElement(setExps[expName], this.element)
+    );
+    return (
+      getExperimentBranch(this.win, AD_SIZE_OPTIMIZATION_EXP.branch) ==
+      AD_SIZE_OPTIMIZATION_EXP.experiment
+    );
   }
 
   /** @override */
@@ -307,6 +446,26 @@ export class AmpAdNetworkAdsenseImpl extends AmpA4A {
     );
   }
 
+  /**
+   * @return {string} ad client ID for the current ad unit.
+   */
+  getAdClientId() {
+    const adClientId = (
+      this.element.getAttribute('data-ad-client') || ''
+    ).toLowerCase();
+    if (!/^ca-/i.test(adClientId)) {
+      return `ca-${adClientId}`;
+    }
+    return adClientId;
+  }
+
+  /**
+   * @return {string} ad size optimization storage key.
+   */
+  getAdSizeOptimizationStorageKey() {
+    return `aas-${this.getAdClientId()}`;
+  }
+
   /** @override */
   getAdUrl(consentState) {
     if (
@@ -320,12 +479,7 @@ export class AmpAdNetworkAdsenseImpl extends AmpA4A {
     // validateData, from 3p/3p/js, after moving it someplace common.
     const startTime = Date.now();
     const global = this.win;
-    let adClientId = this.element.getAttribute('data-ad-client');
-    // Ensure client id format: lower case with 'ca-' prefix.
-    adClientId = adClientId.toLowerCase();
-    if (adClientId.substring(0, 3) != 'ca-') {
-      adClientId = 'ca-' + adClientId;
-    }
+    const adClientId = this.getAdClientId();
     const adTestOn =
       this.element.getAttribute('data-adtest') ||
       isInManualExperiment(this.element);
@@ -511,6 +665,9 @@ export class AmpAdNetworkAdsenseImpl extends AmpA4A {
   /** @override */
   onCreativeRender(creativeMetaData) {
     super.onCreativeRender(creativeMetaData);
+    if (this.isInAdSizeOptimizationExperimentBranch()) {
+      this.attachSettingsListener();
+    }
     this.isAmpCreative_ = !!creativeMetaData;
     if (
       creativeMetaData &&
diff --git a/extensions/amp-ad-network-adsense-impl/0.1/test/test-amp-ad-network-adsense-impl.js b/extensions/amp-ad-network-adsense-impl/0.1/test/test-amp-ad-network-adsense-impl.js
index 6af159d081fba..9e0cf68c30503 100644
--- a/extensions/amp-ad-network-adsense-impl/0.1/test/test-amp-ad-network-adsense-impl.js
+++ b/extensions/amp-ad-network-adsense-impl/0.1/test/test-amp-ad-network-adsense-impl.js
@@ -19,12 +19,13 @@
 // always available for them. However, when we test an impl in isolation,
 // AmpAd is not loaded already, so we need to load it separately.
 import '../../../amp-ad/0.1/amp-ad';
-import {AmpA4A} from '../../../amp-a4a/0.1/amp-a4a';
-import {AmpAd} from '../../../amp-ad/0.1/amp-ad';
 import {
+  AD_SIZE_OPTIMIZATION_EXP,
   AmpAdNetworkAdsenseImpl,
   resetSharedState,
-} from '../amp-ad-network-adsense-impl'; // eslint-disable-line no-unused-vars
+} from '../amp-ad-network-adsense-impl';
+import {AmpA4A} from '../../../amp-a4a/0.1/amp-a4a';
+import {AmpAd} from '../../../amp-ad/0.1/amp-ad'; // eslint-disable-line no-unused-vars
 import {
   AmpAdXOriginIframeHandler, // eslint-disable-line no-unused-vars
 } from '../../../amp-ad/0.1/amp-ad-xorigin-iframe-handler';
@@ -62,6 +63,7 @@ describes.realWin(
     let impl;
     let element;
     let isResponsiveStub;
+    let storageMock;
 
     beforeEach(() => {
       win = env.win;
@@ -90,6 +92,18 @@ describes.realWin(
       impl = new AmpAdNetworkAdsenseImpl(element);
       impl.win['goog_identity_prom'] = Promise.resolve({});
       isResponsiveStub = sandbox.stub(impl, 'isResponsive_');
+      // Make tests faster by completing timeouts instantly.
+      sandbox.stub(Services, 'timerFor').returns({
+        timeoutPromise: (unused, promise) => {
+          if (promise) {
+            return promise;
+          }
+          return Promise.reject(new Error('No token'));
+        },
+      });
+      return Services.storageForDoc(element).then(storage => {
+        storageMock = sandbox.mock(storage);
+      });
     });
 
     /**
@@ -422,6 +436,49 @@ describes.realWin(
         );
         expect(impl.iframe.id).to.equal('google_ads_iframe_3');
       });
+
+      it('should write auto ad size data to localstorage', function*() {
+        storageMock
+          .expects('set')
+          .withExactArgs('aas-ca-adsense', true)
+          .returns(Promise.resolve())
+          .once();
+
+        forceExperimentBranch(
+          impl.win,
+          AD_SIZE_OPTIMIZATION_EXP.branch,
+          AD_SIZE_OPTIMIZATION_EXP.experiment
+        );
+        impl.iframe = {
+          contentWindow: window,
+          nodeType: 1,
+          style: {},
+        };
+        impl.element.setAttribute('data-ad-client', 'ca-adsense');
+
+        impl.size_ = {width: 123, height: 456};
+
+        impl.onCreativeRender();
+
+        const data = {
+          'googMsgType': 'adsense-settings',
+          'adClient': 'ca-adsense',
+          'enableAutoAdSize': '1',
+        };
+        let promiseResolver;
+        const savePromise = new Promise(resolve => {
+          promiseResolver = resolve;
+        });
+        impl.onAutoAdSizeSettingsStored_ = () => {
+          promiseResolver();
+        };
+
+        win.postMessage(data, '*');
+
+        yield savePromise;
+
+        storageMock.verify();
+      });
     });
 
     describe('centering', () => {
@@ -878,6 +935,7 @@ describes.realWin(
 
       function constructImpl(config) {
         config.type = 'adsense';
+        config['data-ad-client'] = 'ca-adsense';
         element = createElementWithAttributes(doc, 'amp-ad', config);
         iframe = env.win.document.createElement('iframe');
         element.appendChild(iframe);
@@ -895,12 +953,20 @@ describes.realWin(
         return impl;
       }
 
-      it('should do nothing for non-responsive', () => {
+      it('should do nothing for non-responsive', function*() {
         const adsense = constructImpl({
           width: '320',
           height: '150',
         });
-        expect(adsense.buildCallback()).to.be.undefined;
+        env.sandbox
+          .stub(adsense, 'attemptChangeSize')
+          .returns(Promise.resolve());
+
+        const promise = adsense.buildCallback();
+        expect(promise).to.exist;
+        yield promise;
+
+        expect(adsense.attemptChangeSize).to.not.be.called;
       });
 
       it('should schedule a resize for responsive', function*() {
@@ -953,6 +1019,117 @@ describes.realWin(
           VIEWPORT_WIDTH
         );
       });
+
+      describe('for publisher opted in to auto ad size optimization', () => {
+        beforeEach(() => {
+          storageMock
+            .expects('get')
+            .withExactArgs('aas-ca-adsense')
+            .returns(Promise.resolve(true));
+        });
+
+        it('does nothing if experiment is disabled', function*() {
+          forceExperimentBranch(
+            impl.win,
+            AD_SIZE_OPTIMIZATION_EXP.branch,
+            AD_SIZE_OPTIMIZATION_EXP.control
+          );
+          const adsense = constructImpl({
+            width: '320',
+            height: '150',
+          });
+          env.sandbox
+            .stub(adsense, 'attemptChangeSize')
+            .returns(Promise.resolve());
+
+          const promise = adsense.buildCallback();
+          expect(promise).to.exist;
+          yield promise;
+
+          expect(adsense.attemptChangeSize).to.not.be.called;
+          expect(adsense.element.hasAttribute('data-auto-format')).to.be.false;
+        });
+
+        it('does nothing if ad unit is responsive already', function*() {
+          forceExperimentBranch(
+            impl.win,
+            AD_SIZE_OPTIMIZATION_EXP.branch,
+            AD_SIZE_OPTIMIZATION_EXP.experiment
+          );
+          const adsense = constructImpl({
+            width: '100vw',
+            height: '100',
+            'data-auto-format': 'mcrspv',
+          });
+          env.sandbox
+            .stub(adsense, 'attemptChangeSize')
+            .returns(Promise.resolve());
+          const promise = adsense.buildCallback();
+          expect(promise).to.exist;
+          yield promise;
+          expect(adsense.attemptChangeSize).to.be.calledWith(
+            1386,
+            VIEWPORT_WIDTH
+          );
+        });
+
+        it('upgrades manual ad units to responsive if experiment is enabled', function*() {
+          forceExperimentBranch(
+            impl.win,
+            AD_SIZE_OPTIMIZATION_EXP.branch,
+            AD_SIZE_OPTIMIZATION_EXP.experiment
+          );
+          const adsense = constructImpl({
+            width: '320',
+            height: '150',
+          });
+          env.sandbox
+            .stub(adsense, 'attemptChangeSize')
+            .returns(Promise.resolve());
+
+          const promise = adsense.buildCallback();
+          expect(promise).to.exist;
+          yield promise;
+
+          expect(adsense.element.getAttribute('data-auto-format')).to.be.equal(
+            'rspv'
+          );
+          expect(adsense.attemptChangeSize).to.be.calledWith(
+            300,
+            VIEWPORT_WIDTH
+          );
+        });
+      });
+      describe('for publisher not opted in to auto ad size optimization', () => {
+        beforeEach(() => {
+          storageMock
+            .expects('get')
+            .withExactArgs('aas-ca-adsense')
+            .returns(Promise.resolve(false));
+        });
+
+        it('does not upgrade manual ad units to responsive if experiment is enabled', function*() {
+          forceExperimentBranch(
+            impl.win,
+            AD_SIZE_OPTIMIZATION_EXP.branch,
+            AD_SIZE_OPTIMIZATION_EXP.experiment
+          );
+          const adsense = constructImpl({
+            width: '320',
+            height: '150',
+          });
+          env.sandbox
+            .stub(adsense, 'attemptChangeSize')
+            .returns(Promise.resolve());
+
+          const promise = adsense.buildCallback();
+          expect(promise).to.exist;
+          yield promise;
+
+          expect(adsense.attemptChangeSize).to.not.be.called;
+          expect(adsense.element.hasAttribute('data-auto-format')).to.be.false;
+        });
+      });
     });
 
     describe('#onLayoutMeasure', () => {
@@ -1288,5 +1465,24 @@ describes.realWin(
         expect(letCreativeTriggerRenderStart).to.equal(false);
       });
     });
+
+    describe('#getAdClientId', () => {
+      it('should be same as data-ad-client if it has a ca- prefix', () => {
+        element.setAttribute('data-ad-client', 'ca-pub-123');
+        expect(impl.getAdClientId()).to.equal('ca-pub-123');
+      });
+      it("should have a ca- prefix if data-ad-client doesn't", () => {
+        element.setAttribute('data-ad-client', 'pub-456');
+        expect(impl.getAdClientId()).to.equal('ca-pub-456');
+      });
+    });
+    describe('#getAdSizeOptimizationStorageKey', () => {
+      it('should be the data-ad-client attribute prefixed by aas-', () => {
+        element.setAttribute('data-ad-client', 'ca-pub-123');
+        expect(impl.getAdSizeOptimizationStorageKey()).to.equal(
+          'aas-ca-pub-123'
+        );
+      });
+    });
   }
 );
diff --git a/spec/amp-localstorage.md b/spec/amp-localstorage.md
index 32f3dbeac9592..281a9f613325a 100644
--- a/spec/amp-localstorage.md
+++ b/spec/amp-localstorage.md
@@ -54,6 +54,7 @@ The following AMP components and service are using the localStorage.
 
 ** Please add all future usage to the following list
 
+- `<amp-ad-network-adsense-impl>` : Store publisher ad size opt in status.
 - `<amp-app-banner>` : Store the user decision on dismiss banner
 - `<amp-user-notification>` : Store the user decision on dismiss notification
 - `<amp-consent>` : Store the user decision and granular information on consent