diff --git a/spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonDecoder.java b/spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonDecoder.java
index 58f582bb1e8..64e2d43ddb4 100644
--- a/spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonDecoder.java
+++ b/spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonDecoder.java
@@ -21,6 +21,7 @@ import java.util.Map;
 
 import kotlinx.serialization.KSerializer;
 import kotlinx.serialization.SerializersKt;
+import kotlinx.serialization.descriptors.PolymorphicKind;
 import kotlinx.serialization.json.Json;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
@@ -39,7 +40,9 @@ import org.springframework.util.MimeType;
  * Decode a byte stream into JSON and convert to Object's with
  * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
  *
- * <p>This decoder can be used to bind {@code @Serializable} Kotlin classes.
+ * <p>This decoder can be used to bind {@code @Serializable} Kotlin classes,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphic serialization</a>
+ * is not supported.
  * It supports {@code application/json} and {@code application/*+json} with
  * various character sets, {@code UTF-8} being the default.
  *
@@ -132,6 +135,9 @@ public class KotlinSerializationJsonDecoder extends AbstractDecoder<Object> {
 		KSerializer<Object> serializer = serializerCache.get(type);
 		if (serializer == null) {
 			serializer = SerializersKt.serializer(type);
+			if (serializer.getDescriptor().getKind().equals(PolymorphicKind.OPEN.INSTANCE)) {
+				throw new UnsupportedOperationException("Open polymorphic serialization is not supported yet");
+			}
 			serializerCache.put(type, serializer);
 		}
 		return serializer;
diff --git a/spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonEncoder.java b/spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonEncoder.java
index cd577d4c828..3c30b2b0e00 100644
--- a/spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonEncoder.java
+++ b/spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonEncoder.java
@@ -22,6 +22,7 @@ import java.util.Map;
 
 import kotlinx.serialization.KSerializer;
 import kotlinx.serialization.SerializersKt;
+import kotlinx.serialization.descriptors.PolymorphicKind;
 import kotlinx.serialization.json.Json;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
@@ -42,7 +43,9 @@ import org.springframework.util.MimeType;
  * Encode from an {@code Object} stream to a byte stream of JSON objects using
  * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
  *
- * <p>This encoder can be used to bind {@code @Serializable} Kotlin classes.
+ * <p>This encoder can be used to bind {@code @Serializable} Kotlin classes,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphic serialization</a>
+ * is not supported.
  * It supports {@code application/json} and {@code application/*+json} with
  * various character sets, {@code UTF-8} being the default.
  *
@@ -120,6 +123,9 @@ public class KotlinSerializationJsonEncoder extends AbstractEncoder<Object> {
 		KSerializer<Object> serializer = serializerCache.get(type);
 		if (serializer == null) {
 			serializer = SerializersKt.serializer(type);
+			if (serializer.getDescriptor().getKind().equals(PolymorphicKind.OPEN.INSTANCE)) {
+				throw new UnsupportedOperationException("Open polymorphic serialization is not supported yet");
+			}
 			serializerCache.put(type, serializer);
 		}
 		return serializer;
diff --git a/spring-web/src/main/java/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverter.java b/spring-web/src/main/java/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverter.java
index 7149868217a..6025255e19e 100644
--- a/spring-web/src/main/java/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverter.java
+++ b/spring-web/src/main/java/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverter.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,7 @@ import java.util.Map;
 import kotlinx.serialization.KSerializer;
 import kotlinx.serialization.SerializationException;
 import kotlinx.serialization.SerializersKt;
+import kotlinx.serialization.descriptors.PolymorphicKind;
 import kotlinx.serialization.json.Json;
 
 import org.springframework.core.GenericTypeResolver;
@@ -43,7 +44,9 @@ import org.springframework.util.StreamUtils;
  * that can read and write JSON using
  * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
  *
- * <p>This converter can be used to bind {@code @Serializable} Kotlin classes.
+ * <p>This converter can be used to bind {@code @Serializable} Kotlin classes,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphic serialization</a>
+ * is not supported.
  * It supports {@code application/json} and {@code application/*+json} with
  * various character sets, {@code UTF-8} being the default.
  *
@@ -182,6 +185,9 @@ public class KotlinSerializationJsonHttpMessageConverter extends AbstractGeneric
 		KSerializer<Object> serializer = serializerCache.get(type);
 		if (serializer == null) {
 			serializer = SerializersKt.serializer(type);
+			if (serializer.getDescriptor().getKind().equals(PolymorphicKind.OPEN.INSTANCE)) {
+				throw new UnsupportedOperationException("Open polymorphic serialization is not supported yet");
+			}
 			serializerCache.put(type, serializer);
 		}
 		return serializer;
diff --git a/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonDecoderTests.kt b/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonDecoderTests.kt
index f1489814db1..d333bd0a86b 100644
--- a/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonDecoderTests.kt
+++ b/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonDecoderTests.kt
@@ -19,6 +19,7 @@ package org.springframework.http.codec.json
 import kotlinx.serialization.Serializable
 import org.assertj.core.api.Assertions
 import org.junit.jupiter.api.Test
+import org.springframework.core.Ordered
 import org.springframework.core.ResolvableType
 import org.springframework.core.io.buffer.DataBuffer
 import org.springframework.core.testfixture.codec.AbstractDecoderTests
@@ -58,6 +59,7 @@ class KotlinSerializationJsonDecoderTests : AbstractDecoderTests<KotlinSerializa
 		Assertions.assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Pojo::class.java), MediaType.APPLICATION_JSON)).isTrue()
 		Assertions.assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_JSON)).isTrue()
 		Assertions.assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_PDF)).isFalse()
+		Assertions.assertThat(decoder.canDecode(ResolvableType.forClass(Ordered::class.java), MediaType.APPLICATION_JSON)).isFalse()
 	}
 
 	@Test
diff --git a/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt b/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt
index 7252884acf0..52aab80fe6a 100644
--- a/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt
+++ b/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt
@@ -19,6 +19,7 @@ package org.springframework.http.codec.json
 import kotlinx.serialization.Serializable
 import org.assertj.core.api.Assertions
 import org.junit.jupiter.api.Test
+import org.springframework.core.Ordered
 import org.springframework.core.ResolvableType
 import org.springframework.core.io.buffer.DataBuffer
 import org.springframework.core.io.buffer.DataBufferUtils
@@ -91,6 +92,7 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 		Assertions.assertThat(encoder.canEncode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_XML)).isFalse()
 		val sseType = ResolvableType.forClass(ServerSentEvent::class.java)
 		Assertions.assertThat(encoder.canEncode(sseType, MediaType.APPLICATION_JSON)).isFalse()
+		Assertions.assertThat(encoder.canEncode(ResolvableType.forClass(Ordered::class.java), MediaType.APPLICATION_JSON)).isFalse()
 	}
 
 
diff --git a/spring-web/src/test/kotlin/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverterTests.kt b/spring-web/src/test/kotlin/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverterTests.kt
index c9b6cf44dbf..3fe257c21fb 100644
--- a/spring-web/src/test/kotlin/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverterTests.kt
+++ b/spring-web/src/test/kotlin/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverterTests.kt
@@ -20,6 +20,7 @@ import kotlinx.serialization.Serializable
 import org.assertj.core.api.Assertions.assertThat
 import org.assertj.core.api.Assertions.assertThatExceptionOfType
 import org.junit.jupiter.api.Test
+import org.springframework.core.Ordered
 import org.springframework.http.MediaType
 import org.springframework.http.MockHttpInputMessage
 import org.springframework.http.MockHttpOutputMessage
@@ -48,14 +49,18 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		assertThat(converter.canRead(String::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canRead(NotSerializableBean::class.java, MediaType.APPLICATION_JSON)).isFalse()
 
-		assertThat(converter.canRead(Map::class.java, MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canRead(List::class.java, MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canRead(Set::class.java, MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canRead(Map::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canRead(typeTokenOf<Map<String, SerializableBean>>(), Map::class.java, MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canRead(List::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canRead(typeTokenOf<List<SerializableBean>>(), List::class.java, MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canRead(Set::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canRead(typeTokenOf<Set<SerializableBean>>(), Set::class.java, MediaType.APPLICATION_JSON)).isTrue()
 
-		assertThat(converter.canRead(typeTokenOf<List<Int>>(), null, MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canRead(typeTokenOf<List<SerializableBean>>(), null, MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canRead(typeTokenOf<ArrayList<Int>>(), null, MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canRead(typeTokenOf<List<Int>>(), null, MediaType.APPLICATION_PDF)).isFalse()
+		assertThat(converter.canRead(typeTokenOf<List<Int>>(), List::class.java, MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canRead(typeTokenOf<ArrayList<Int>>(), List::class.java, MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canRead(typeTokenOf<List<Int>>(), List::class.java, MediaType.APPLICATION_PDF)).isFalse()
+
+		assertThat(converter.canRead(typeTokenOf<Ordered>(), Ordered::class.java, MediaType.APPLICATION_JSON)).isFalse()
 	}
 
 	@Test
@@ -65,14 +70,18 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		assertThat(converter.canWrite(String::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canWrite(NotSerializableBean::class.java, MediaType.APPLICATION_JSON)).isFalse()
 
-		assertThat(converter.canWrite(Map::class.java, MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canWrite(List::class.java, MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canWrite(Set::class.java, MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canWrite(Map::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canWrite(typeTokenOf<Map<String, SerializableBean>>(), Map::class.java, MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canWrite(List::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canWrite(typeTokenOf<List<SerializableBean>>(), List::class.java, MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canWrite(Set::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canWrite(typeTokenOf<Set<SerializableBean>>(), Set::class.java, MediaType.APPLICATION_JSON)).isTrue()
 
 		assertThat(converter.canWrite(typeTokenOf<List<Int>>(), List::class.java, MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canWrite(typeTokenOf<List<SerializableBean>>(), List::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canWrite(typeTokenOf<ArrayList<Int>>(), List::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canWrite(typeTokenOf<List<Int>>(), List::class.java, MediaType.APPLICATION_PDF)).isFalse()
+
+		assertThat(converter.canWrite(typeTokenOf<Ordered>(), Ordered::class.java, MediaType.APPLICATION_JSON)).isFalse()
 	}
 
 	@Test