diff --git a/modules/criteoBidAdapter.js b/modules/criteoBidAdapter.js
index 5d137c994fe0..60ad15c1d9b9 100644
--- a/modules/criteoBidAdapter.js
+++ b/modules/criteoBidAdapter.js
@@ -1,14 +1,14 @@
-import { loadExternalScript } from '../src/adloader.js';
-import { registerBidder } from '../src/adapters/bidderFactory.js';
-import { config } from '../src/config.js';
-import { BANNER, NATIVE, VIDEO } from '../src/mediaTypes.js';
+import {loadExternalScript} from '../src/adloader.js';
+import {registerBidder} from '../src/adapters/bidderFactory.js';
+import {config} from '../src/config.js';
+import {BANNER, NATIVE, VIDEO} from '../src/mediaTypes.js';
 import * as utils from '../src/utils.js';
 import find from 'core-js/library/fn/array/find.js';
-import { verify } from 'criteo-direct-rsa-validate/build/verify.js';
-import { getStorageManager } from '../src/storageManager.js';
+import {verify} from 'criteo-direct-rsa-validate/build/verify.js';
+import {getStorageManager} from '../src/storageManager.js';
 
 const GVLID = 91;
-export const ADAPTER_VERSION = 29;
+export const ADAPTER_VERSION = 31;
 const BIDDER_CODE = 'criteo';
 const CDB_ENDPOINT = 'https://bidder.criteo.com/cdb';
 const PROFILE_ID_INLINE = 207;
@@ -274,7 +274,6 @@ function buildCdbRequest(context, bidRequests, bidderRequest) {
         impid: bidRequest.adUnitCode,
         transactionid: bidRequest.transactionId,
         auctionId: bidRequest.auctionId,
-        sizes: getBannerSizes(bidRequest),
       };
       if (bidRequest.params.zoneId) {
         slot.zoneid = bidRequest.params.zoneId;
@@ -293,10 +292,13 @@ function buildCdbRequest(context, bidRequests, bidderRequest) {
         if (!checkNativeSendId(bidRequest)) {
           utils.logWarn(LOG_PREFIX + 'all native assets containing URL should be sent as placeholders with sendId(icon, image, clickUrl, displayUrl, privacyLink, privacyIcon)');
         }
+        slot.sizes = parseSizes(retrieveBannerSizes(bidRequest), parseNativeSize);
+      } else {
+        slot.sizes = parseSizes(retrieveBannerSizes(bidRequest), parseSize);
       }
       if (hasVideoMediaType(bidRequest)) {
         const video = {
-          playersizes: getVideoSizes(bidRequest),
+          playersizes: parseSizes(utils.deepAccess(bidRequest, 'mediaTypes.video.playerSize'), parseSize),
           mimes: bidRequest.mediaTypes.video.mimes,
           protocols: bidRequest.mediaTypes.video.protocols,
           maxduration: bidRequest.mediaTypes.video.maxduration,
@@ -339,24 +341,26 @@ function buildCdbRequest(context, bidRequests, bidderRequest) {
   return request;
 }
 
-function getVideoSizes(bidRequest) {
-  return parseSizes(utils.deepAccess(bidRequest, 'mediaTypes.video.playerSize'));
+function retrieveBannerSizes(bidRequest) {
+  return utils.deepAccess(bidRequest, 'mediaTypes.banner.sizes') || bidRequest.sizes;
 }
 
-function getBannerSizes(bidRequest) {
-  return parseSizes(utils.deepAccess(bidRequest, 'mediaTypes.banner.sizes') || bidRequest.sizes);
+function parseSizes(sizes, parser) {
+  if (Array.isArray(sizes[0])) { // is there several sizes ? (ie. [[728,90],[200,300]])
+    return sizes.map(size => parser(size));
+  }
+  return [parser(sizes)]; // or a single one ? (ie. [728,90])
 }
 
 function parseSize(size) {
   return size[0] + 'x' + size[1];
 }
 
-function parseSizes(sizes) {
-  if (Array.isArray(sizes[0])) { // is there several sizes ? (ie. [[728,90],[200,300]])
-    return sizes.map(size => parseSize(size));
+function parseNativeSize(size) {
+  if (size[0] === undefined && size[1] === undefined) {
+    return '2x2';
   }
-
-  return [parseSize(sizes)]; // or a single one ? (ie. [728,90])
+  return size[0] + 'x' + size[1];
 }
 
 function hasVideoMediaType(bidRequest) {
diff --git a/test/spec/modules/criteoBidAdapter_spec.js b/test/spec/modules/criteoBidAdapter_spec.js
index 042f1a4d61db..6ab0159e9017 100755
--- a/test/spec/modules/criteoBidAdapter_spec.js
+++ b/test/spec/modules/criteoBidAdapter_spec.js
@@ -440,6 +440,62 @@ describe('The Criteo bidding adapter', function () {
       expect(ortbRequest.gdprConsent.version).to.equal(1);
     });
 
+    it('should keep undefined sizes for non native banner', function () {
+      const bidRequests = [
+        {
+          sizes: [[undefined, undefined]],
+          params: {},
+        },
+      ];
+      const request = spec.buildRequests(bidRequests, bidderRequest);
+      const ortbRequest = request.data;
+      expect(ortbRequest.slots[0].sizes).to.have.lengthOf(1);
+      expect(ortbRequest.slots[0].sizes[0]).to.equal('undefinedxundefined');
+    });
+
+    it('should keep undefined size for non native banner', function () {
+      const bidRequests = [
+        {
+          sizes: [undefined, undefined],
+          params: {},
+        },
+      ];
+      const request = spec.buildRequests(bidRequests, bidderRequest);
+      const ortbRequest = request.data;
+      expect(ortbRequest.slots[0].sizes).to.have.lengthOf(1);
+      expect(ortbRequest.slots[0].sizes[0]).to.equal('undefinedxundefined');
+    });
+
+    it('should properly detect and get sizes of native sizeless banner', function () {
+      const bidRequests = [
+        {
+          sizes: [[undefined, undefined]],
+          params: {
+            nativeCallback: function() {}
+          },
+        },
+      ];
+      const request = spec.buildRequests(bidRequests, bidderRequest);
+      const ortbRequest = request.data;
+      expect(ortbRequest.slots[0].sizes).to.have.lengthOf(1);
+      expect(ortbRequest.slots[0].sizes[0]).to.equal('2x2');
+    });
+
+    it('should properly detect and get size of native sizeless banner', function () {
+      const bidRequests = [
+        {
+          sizes: [undefined, undefined],
+          params: {
+            nativeCallback: function() {}
+          },
+        },
+      ];
+      const request = spec.buildRequests(bidRequests, bidderRequest);
+      const ortbRequest = request.data;
+      expect(ortbRequest.slots[0].sizes).to.have.lengthOf(1);
+      expect(ortbRequest.slots[0].sizes[0]).to.equal('2x2');
+    });
+
     it('should properly build a networkId request', function () {
       const bidderRequest = {
         refererInfo: {