diff --git a/src/prebid.js b/src/prebid.js
index 0378f8c27ed..7b1cfd258ec 100644
--- a/src/prebid.js
+++ b/src/prebid.js
@@ -622,7 +622,7 @@ $$PREBID_GLOBAL$$.removeAdUnit = function (adUnitCode) {
  * @alias module:pbjs.requestBids
  */
 $$PREBID_GLOBAL$$.requestBids = (function() {
-  const delegate = hook('async', function ({ bidsBackHandler, timeout, adUnits, adUnitCodes, labels, auctionId, ortb2, metrics } = {}) {
+  const delegate = hook('sync', function ({ bidsBackHandler, timeout, adUnits, adUnitCodes, labels, auctionId, ortb2, metrics } = {}) {
     events.emit(REQUEST_BIDS);
     const cbTimeout = timeout || config.getConfig('bidderTimeout');
     logInfo('Invoking $$PREBID_GLOBAL$$.requestBids', arguments);
@@ -646,7 +646,7 @@ $$PREBID_GLOBAL$$.requestBids = (function() {
   });
 })();
 
-export const startAuction = hook('async', function ({ bidsBackHandler, timeout: cbTimeout, adUnits, adUnitCodes, labels, auctionId, ortb2Fragments, metrics } = {}) {
+export const startAuction = hook('sync', function ({ bidsBackHandler, timeout: cbTimeout, adUnits, adUnitCodes, labels, auctionId, ortb2Fragments, metrics } = {}) {
   const s2sBidders = getS2SBidderSet(config.getConfig('s2sConfig') || []);
   adUnits = useMetrics(metrics).measureTime('requestBids.validate', () => checkAdUnitSetup(adUnits));
 
@@ -658,77 +658,82 @@ export const startAuction = hook('async', function ({ bidsBackHandler, timeout:
     adUnitCodes = adUnits && adUnits.map(unit => unit.code);
   }
 
-  /*
-   * for a given adunit which supports a set of mediaTypes
-   * and a given bidder which supports a set of mediaTypes
-   * a bidder is eligible to participate on the adunit
-   * if it supports at least one of the mediaTypes on the adunit
-   */
-  adUnits.forEach(adUnit => {
-    // get the adunit's mediaTypes, defaulting to banner if mediaTypes isn't present
-    const adUnitMediaTypes = Object.keys(adUnit.mediaTypes || { 'banner': 'banner' });
-
-    // get the bidder's mediaTypes
-    const allBidders = adUnit.bids.map(bid => bid.bidder);
-    const bidderRegistry = adapterManager.bidderRegistry;
-
-    const bidders = allBidders.filter(bidder => !s2sBidders.has(bidder));
-
-    const tid = adUnit.ortb2Imp?.ext?.tid || generateUUID();
-    adUnit.transactionId = tid;
-    // Populate ortb2Imp.ext.tid with transactionId. Specifying a transaction ID per item in the ortb impression array, lets multiple transaction IDs be transmitted in a single bid request.
-    deepSetValue(adUnit, 'ortb2Imp.ext.tid', tid);
-
-    bidders.forEach(bidder => {
-      const adapter = bidderRegistry[bidder];
-      const spec = adapter && adapter.getSpec && adapter.getSpec();
-      // banner is default if not specified in spec
-      const bidderMediaTypes = (spec && spec.supportedMediaTypes) || ['banner'];
-
-      // check if the bidder's mediaTypes are not in the adUnit's mediaTypes
-      const bidderEligible = adUnitMediaTypes.some(type => includes(bidderMediaTypes, type));
-      if (!bidderEligible) {
-        // drop the bidder from the ad unit if it's not compatible
-        logWarn(unsupportedBidderMessage(adUnit, bidder));
-        adUnit.bids = adUnit.bids.filter(bid => bid.bidder !== bidder);
-      } else {
-        adunitCounter.incrementBidderRequestsCounter(adUnit.code, bidder);
+  return new Promise((resolve) => {
+    function auctionDone(bids, timedOut, auctionId) {
+      if (typeof bidsBackHandler === 'function') {
+        try {
+          bidsBackHandler(bids, timedOut, auctionId);
+        } catch (e) {
+          logError('Error executing bidsBackHandler', null, e);
+        }
       }
+      resolve({bids, timedOut, auctionId});
+    }
+
+    /*
+     * for a given adunit which supports a set of mediaTypes
+     * and a given bidder which supports a set of mediaTypes
+     * a bidder is eligible to participate on the adunit
+     * if it supports at least one of the mediaTypes on the adunit
+     */
+    adUnits.forEach(adUnit => {
+      // get the adunit's mediaTypes, defaulting to banner if mediaTypes isn't present
+      const adUnitMediaTypes = Object.keys(adUnit.mediaTypes || { 'banner': 'banner' });
+
+      // get the bidder's mediaTypes
+      const allBidders = adUnit.bids.map(bid => bid.bidder);
+      const bidderRegistry = adapterManager.bidderRegistry;
+
+      const bidders = allBidders.filter(bidder => !s2sBidders.has(bidder));
+
+      const tid = adUnit.ortb2Imp?.ext?.tid || generateUUID();
+      adUnit.transactionId = tid;
+      // Populate ortb2Imp.ext.tid with transactionId. Specifying a transaction ID per item in the ortb impression array, lets multiple transaction IDs be transmitted in a single bid request.
+      deepSetValue(adUnit, 'ortb2Imp.ext.tid', tid);
+
+      bidders.forEach(bidder => {
+        const adapter = bidderRegistry[bidder];
+        const spec = adapter && adapter.getSpec && adapter.getSpec();
+        // banner is default if not specified in spec
+        const bidderMediaTypes = (spec && spec.supportedMediaTypes) || ['banner'];
+
+        // check if the bidder's mediaTypes are not in the adUnit's mediaTypes
+        const bidderEligible = adUnitMediaTypes.some(type => includes(bidderMediaTypes, type));
+        if (!bidderEligible) {
+          // drop the bidder from the ad unit if it's not compatible
+          logWarn(unsupportedBidderMessage(adUnit, bidder));
+          adUnit.bids = adUnit.bids.filter(bid => bid.bidder !== bidder);
+        } else {
+          adunitCounter.incrementBidderRequestsCounter(adUnit.code, bidder);
+        }
+      });
+      adunitCounter.incrementRequestsCounter(adUnit.code);
     });
-    adunitCounter.incrementRequestsCounter(adUnit.code);
-  });
 
-  if (!adUnits || adUnits.length === 0) {
-    logMessage('No adUnits configured. No bids requested.');
-    if (typeof bidsBackHandler === 'function') {
-      // executeCallback, this will only be called in case of first request
-      try {
-        bidsBackHandler();
-      } catch (e) {
-        logError('Error executing bidsBackHandler', null, e);
+    if (!adUnits || adUnits.length === 0) {
+      logMessage('No adUnits configured. No bids requested.');
+      auctionDone();
+    } else {
+      const auction = auctionManager.createAuction({
+        adUnits,
+        adUnitCodes,
+        callback: auctionDone,
+        cbTimeout,
+        labels,
+        auctionId,
+        ortb2Fragments,
+        metrics,
+      });
+
+      let adUnitsLen = adUnits.length;
+      if (adUnitsLen > 15) {
+        logInfo(`Current auction ${auction.getAuctionId()} contains ${adUnitsLen} adUnits.`, adUnits);
       }
-    }
-    return;
-  }
 
-  const auction = auctionManager.createAuction({
-    adUnits,
-    adUnitCodes,
-    callback: bidsBackHandler,
-    cbTimeout,
-    labels,
-    auctionId,
-    ortb2Fragments,
-    metrics,
+      adUnitCodes.forEach(code => targeting.setLatestAuctionForAdUnit(code, auction.getAuctionId()));
+      auction.callBids();
+    }
   });
-
-  let adUnitsLen = adUnits.length;
-  if (adUnitsLen > 15) {
-    logInfo(`Current auction ${auction.getAuctionId()} contains ${adUnitsLen} adUnits.`, adUnits);
-  }
-
-  adUnitCodes.forEach(code => targeting.setLatestAuctionForAdUnit(code, auction.getAuctionId()));
-  auction.callBids();
 }, 'startAuction');
 
 export function executeCallbacks(fn, reqBidsConfigObj) {
diff --git a/test/spec/unit/pbjs_api_spec.js b/test/spec/unit/pbjs_api_spec.js
index 4c1f5c3b455..e83608325d8 100644
--- a/test/spec/unit/pbjs_api_spec.js
+++ b/test/spec/unit/pbjs_api_spec.js
@@ -21,7 +21,8 @@ import {hook} from '../../../src/hook.js';
 import {reset as resetDebugging} from '../../../src/debugging.js';
 import $$PREBID_GLOBAL$$ from 'src/prebid.js';
 import {resetAuctionState} from 'src/auction.js';
-
+import {stubAuctionIndex} from '../../helpers/indexStub.js';
+import {createBid} from '../../../src/bidfactory.js';
 var assert = require('chai').assert;
 var expect = require('chai').expect;
 
@@ -1467,7 +1468,6 @@ describe('Unit: Prebid Module', function () {
     after(function () {
       clock.restore();
     });
-    let bidsBackHandlerStub = sinon.stub();
 
     const BIDDER_CODE = 'sampleBidder';
     let bids = [{
@@ -1504,11 +1504,12 @@ describe('Unit: Prebid Module', function () {
       'start': 1000
     }];
 
+    let spec, indexStub, auction, completeAuction;
+
     beforeEach(function () {
       logMessageSpy = sinon.spy(utils, 'logMessage');
       makeRequestsStub = sinon.stub(adapterManager, 'makeBidRequests');
       makeRequestsStub.returns(bidRequests);
-
       adUnits = [{
         code: 'adUnit-code',
         mediaTypes: {
@@ -1516,45 +1517,53 @@ describe('Unit: Prebid Module', function () {
             sizes: [[300, 250]]
           }
         },
+        transactionId: 'mock-tid',
         bids: [
           {bidder: BIDDER_CODE, params: {placementId: 'id'}},
         ]
       }];
-      let adUnitCodes = ['adUnit-code'];
-      let auction = auctionModule.newAuction({
-        adUnits,
-        adUnitCodes,
-        callback: bidsBackHandlerStub,
-        cbTimeout: 2000
-      });
-      let createAuctionStub = sinon.stub(auctionModule, 'newAuction');
-      createAuctionStub.returns(auction);
-    });
-
-    afterEach(function () {
-      clock.restore();
-      adapterManager.makeBidRequests.restore();
-      auctionModule.newAuction.restore();
-      utils.logMessage.restore();
-    });
-
-    it('should execute callback after timeout', function () {
-      let spec = {
+      indexStub = sinon.stub(auctionManager, 'index');
+      indexStub.get(() => stubAuctionIndex({adUnits, bidRequests}))
+      sinon.stub(adapterManager, 'callBids').callsFake((_, bidrequests, addBidResponse, adapterDone) => {
+        completeAuction = (bidsReceived) => {
+          bidsReceived.forEach((bid) => addBidResponse(bid.adUnitCode, Object.assign(createBid(), bid)));
+          bidRequests.forEach((req) => adapterDone.call(req));
+        }
+      })
+      const origNewAuction = auctionModule.newAuction;
+      sinon.stub(auctionModule, 'newAuction').callsFake(function (opts) {
+        auction = origNewAuction(opts);
+        return auction;
+      })
+      spec = {
         code: BIDDER_CODE,
         isBidRequestValid: sinon.stub(),
         buildRequests: sinon.stub(),
         interpretResponse: sinon.stub(),
         getUserSyncs: sinon.stub(),
-        onTimeout: sinon.stub()
+        onTimeout: sinon.stub(),
+        onSetTargeting: sinon.stub(),
       };
 
       registerBidder(spec);
       spec.buildRequests.returns([{'id': 123, 'method': 'POST'}]);
       spec.isBidRequestValid.returns(true);
       spec.interpretResponse.returns(bids);
+    });
+
+    afterEach(function () {
+      clock.restore();
+      adapterManager.makeBidRequests.restore();
+      adapterManager.callBids.restore();
+      indexStub.restore();
+      auction.getBidsReceived = () => [];
+      auctionModule.newAuction.restore();
+      utils.logMessage.restore();
+    });
 
+    it('should execute callback after timeout', function () {
       let requestObj = {
-        bidsBackHandler: null, // does not need to be defined because of newAuction mock in beforeEach
+        bidsBackHandler: sinon.stub(),
         timeout: 2000,
         adUnits: adUnits
       };
@@ -1567,26 +1576,13 @@ describe('Unit: Prebid Module', function () {
       clock.tick(1);
       assert.ok(logMessageSpy.calledWith(sinon.match(re)), 'executeCallback called');
 
-      expect(bidsBackHandlerStub.getCall(0).args[1]).to.equal(true,
+      expect(requestObj.bidsBackHandler.getCall(0).args[1]).to.equal(true,
         'bidsBackHandler should be called with timedOut=true');
 
       sinon.assert.called(spec.onTimeout);
     });
 
-    it('should execute callback after setTargeting', function () {
-      let spec = {
-        code: BIDDER_CODE,
-        isBidRequestValid: sinon.stub(),
-        buildRequests: sinon.stub(),
-        interpretResponse: sinon.stub(),
-        onSetTargeting: sinon.stub()
-      };
-
-      registerBidder(spec);
-      spec.buildRequests.returns([{'id': 123, 'method': 'POST'}]);
-      spec.isBidRequestValid.returns(true);
-      spec.interpretResponse.returns(bids);
-
+    it('should execute `onSetTargeting` after setTargetingForGPTAsync', function () {
       const bidId = 1;
       const auctionId = 1;
       let adResponse = Object.assign({
@@ -1595,6 +1591,7 @@ describe('Unit: Prebid Module', function () {
         width: 300,
         height: 250,
         adUnitCode: bidRequests[0].bids[0].adUnitCode,
+        transactionId: 'mock-tid',
         adserverTargeting: {
           'hb_bidder': BIDDER_CODE,
           'hb_adid': bidId,
@@ -1603,20 +1600,76 @@ describe('Unit: Prebid Module', function () {
         },
         bidder: bids[0].bidderCode,
       }, bids[0]);
-      auction.getBidsReceived = function() { return [adResponse]; }
-      auction.getAuctionId = () => auctionId;
 
       let requestObj = {
-        bidsBackHandler: null, // does not need to be defined because of newAuction mock in beforeEach
+        bidsBackHandler: null,
         timeout: 2000,
         adUnits: adUnits
       };
 
       $$PREBID_GLOBAL$$.requestBids(requestObj);
+      completeAuction([adResponse]);
       $$PREBID_GLOBAL$$.setTargetingForGPTAsync();
 
       sinon.assert.called(spec.onSetTargeting);
     });
+
+    describe('returns a promise that resolves', () => {
+      Object.entries({
+        'immediately, without bidsBackHandler': (req) => $$PREBID_GLOBAL$$.requestBids(req),
+        'after bidsBackHandler': (() => {
+          const bidsBackHandler = sinon.stub();
+          return function (req) {
+            return $$PREBID_GLOBAL$$.requestBids({...req, bidsBackHandler}).then(({bids, timedOut, auctionId}) => {
+              sinon.assert.calledWith(bidsBackHandler, bids, timedOut, auctionId);
+              return {bids, timedOut, auctionId};
+            })
+          }
+        })(),
+        'after a bidsBackHandler that throws': (req) => $$PREBID_GLOBAL$$.requestBids({...req, bidsBackHandler: () => { throw new Error() }})
+      }).forEach(([t, requestBids]) => {
+        describe(t, () => {
+          it('with no args, when no adUnits are defined', () => {
+            return requestBids({}).then((res) => {
+              expect(res).to.eql({
+                bids: undefined,
+                timedOut: undefined,
+                auctionId: undefined
+              });
+            });
+          });
+
+          it('on timeout', (done) => {
+            requestBids({
+              auctionId: 'mock-auctionId',
+              adUnits,
+              timeout: 10
+            }).then(({timedOut, bids, auctionId}) => {
+              expect(timedOut).to.be.true;
+              expect(bids).to.eql({});
+              expect(auctionId).to.eql('mock-auctionId');
+              done();
+            });
+            clock.tick(12);
+          });
+
+          it('with auction result', (done) => {
+            const bid = {
+              bidder: 'mock-bidder',
+              adUnitCode: adUnits[0].code,
+              transactionId: adUnits[0].transactionId
+            }
+            requestBids({
+              adUnits,
+            }).then(({bids}) => {
+              sinon.assert.match(bids[bid.adUnitCode].bids[0], bid)
+              done();
+            });
+            completeAuction([bid]);
+          })
+        })
+      })
+    })
   })
 
   describe('requestBids', function () {