diff --git a/src/main/java/org/prebid/server/bidder/pubmatic/PubmaticBidder.java b/src/main/java/org/prebid/server/bidder/pubmatic/PubmaticBidder.java
index 857f212a209..e36c8344504 100644
--- a/src/main/java/org/prebid/server/bidder/pubmatic/PubmaticBidder.java
+++ b/src/main/java/org/prebid/server/bidder/pubmatic/PubmaticBidder.java
@@ -272,7 +272,7 @@ private Imp modifyImp(Imp imp, PubmaticBidderImpExt impExt, String displayManage
         final ObjectNode newExt = makeKeywords(impExt);
 
         final Imp.ImpBuilder impBuilder = imp.toBuilder()
-                .banner(banner != null ? assignSizesIfMissing(banner) : null)
+                .banner(banner != null ? fillBannerSizeFromFormat(banner) : null)
                 .audio(null)
                 .bidfloor(resolveBidFloor(impExtBidder.getKadfloor(), imp.getBidfloor()))
                 .displaymanager(StringUtils.firstNonBlank(imp.getDisplaymanager(), displayManager))
@@ -281,12 +281,24 @@ private Imp modifyImp(Imp imp, PubmaticBidderImpExt impExt, String displayManage
 
         enrichWithAdSlotParameters(impBuilder, impExtBidder.getAdSlot(), banner);
 
-        return impBuilder.build();
+        final Imp result = impBuilder.build();
+        final Banner resultBanner = result.getBanner();
+        // Per OpenRTB 2.6 spec, banner sizes are optional for interstitial impressions.
+        final boolean isInterstitial = Objects.equals(result.getInstl(), 1);
+        if (resultBanner != null && (resultBanner.getW() == null || resultBanner.getH() == null) && !isInterstitial) {
+            throw new PreBidException("No sizes provided for Banner");
+        }
+
+        return result;
     }
 
-    private static Banner assignSizesIfMissing(Banner banner) {
+    private static Banner fillBannerSizeFromFormat(Banner banner) {
         final List<Format> format = banner.getFormat();
-        if ((banner.getW() != null && banner.getH() != null) || CollectionUtils.isEmpty(format)) {
+        if (banner.getW() != null && banner.getH() != null) {
+            return banner;
+        }
+
+        if (CollectionUtils.isEmpty(format)) {
             return banner;
         }
 
diff --git a/src/test/java/org/prebid/server/bidder/pubmatic/PubmaticBidderTest.java b/src/test/java/org/prebid/server/bidder/pubmatic/PubmaticBidderTest.java
index 0d7b50e89a9..a500c6be8d3 100644
--- a/src/test/java/org/prebid/server/bidder/pubmatic/PubmaticBidderTest.java
+++ b/src/test/java/org/prebid/server/bidder/pubmatic/PubmaticBidderTest.java
@@ -587,6 +587,83 @@ public void makeHttpRequestsShouldSetBannerWidthAndHeightFromFormatIfMissedOrigi
                 .containsExactly(100);
     }
 
+    @Test
+    public void makeHttpRequestsShouldReturnErrorIfBannerHasNoSizesAndNoFormat() {
+        // given - non-interstitial banner with no W/H and no format: should error
+        final BidRequest bidRequest = givenBidRequest(
+                impBuilder -> impBuilder.banner(Banner.builder().build()),
+                extImpPubmaticBuilder -> extImpPubmaticBuilder.adSlot("slot"));
+
+        // when
+        final Result<List<HttpRequest<BidRequest>>> result = target.makeHttpRequests(bidRequest);
+
+        // then
+        assertThat(result.getErrors())
+                .containsExactly(BidderError.badInput("No sizes provided for Banner"));
+        assertThat(result.getValue()).isEmpty();
+    }
+
+    @Test
+    public void makeHttpRequestsShouldReturnErrorIfBannerHasOnlyOneDimensionAndNoFormat() {
+        // given - banner with w=300, h=null, no format, adSlot without size: should error
+        final BidRequest bidRequest = givenBidRequest(
+                impBuilder -> impBuilder.banner(Banner.builder().w(300).build()),
+                extImpPubmaticBuilder -> extImpPubmaticBuilder.adSlot("slot"));
+
+        // when
+        final Result<List<HttpRequest<BidRequest>>> result = target.makeHttpRequests(bidRequest);
+
+        // then
+        assertThat(result.getErrors())
+                .containsExactly(BidderError.badInput("No sizes provided for Banner"));
+        assertThat(result.getValue()).isEmpty();
+    }
+
+    @Test
+    public void makeHttpRequestsShouldNotReturnErrorForInterstitialBannerWithNoSizes() {
+        // given - per OpenRTB 2.6, banner sizes are optional for interstitial imps (instl=1)
+        final BidRequest bidRequest = givenBidRequest(
+                impBuilder -> impBuilder
+                        .instl(1)
+                        .banner(Banner.builder().build()),
+                extImpPubmaticBuilder -> extImpPubmaticBuilder.adSlot("slot"));
+
+        // when
+        final Result<List<HttpRequest<BidRequest>>> result = target.makeHttpRequests(bidRequest);
+
+        // then
+        assertThat(result.getErrors()).isEmpty();
+        assertThat(result.getValue()).hasSize(1);
+    }
+
+    @Test
+    public void makeHttpRequestsShouldAllowBannerWithNoFormatWhenAdSlotProvidesSize() {
+        // given
+        final BidRequest bidRequest = givenBidRequest(
+                impBuilder -> impBuilder.banner(Banner.builder().build()));
+
+        // when
+        final Result<List<HttpRequest<BidRequest>>> result = target.makeHttpRequests(bidRequest);
+
+        // then
+        // Default givenBidRequest adSlot is "slot@300x250", so dimensions come from adSlot
+        assertThat(result.getErrors()).isEmpty();
+        assertThat(result.getValue())
+                .extracting(HttpRequest::getPayload)
+                .flatExtracting(BidRequest::getImp)
+                .extracting(Imp::getBanner)
+                .extracting(Banner::getW)
+                .containsExactly(300);
+        assertThat(result.getValue())
+                .extracting(HttpRequest::getPayload)
+                .flatExtracting(BidRequest::getImp)
+                .extracting(Imp::getBanner)
+                .extracting(Banner::getH)
+                .containsExactly(250);
+    }
+
+
+
     @Test
     public void makeHttpRequestsShouldSetTagIdForBannerImpsWithSymbolsFromAdSlotBeforeAtSign() {
         // given