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 64e2d43ddb..304a0cfa7d 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
@@ -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.
@@ -17,11 +17,14 @@
 package org.springframework.http.codec.json;
 
 import java.lang.reflect.Type;
+import java.util.HashSet;
 import java.util.Map;
+import java.util.Set;
 
 import kotlinx.serialization.KSerializer;
 import kotlinx.serialization.SerializersKt;
 import kotlinx.serialization.descriptors.PolymorphicKind;
+import kotlinx.serialization.descriptors.SerialDescriptor;
 import kotlinx.serialization.json.Json;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
@@ -135,7 +138,7 @@ 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)) {
+			if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
 				throw new UnsupportedOperationException("Open polymorphic serialization is not supported yet");
 			}
 			serializerCache.put(type, serializer);
@@ -143,4 +146,18 @@ public class KotlinSerializationJsonDecoder extends AbstractDecoder<Object> {
 		return serializer;
 	}
 
+	private boolean hasPolymorphism(SerialDescriptor descriptor, Set<String> alreadyProcessed) {
+		alreadyProcessed.add(descriptor.getSerialName());
+		if (descriptor.getKind().equals(PolymorphicKind.OPEN.INSTANCE)) {
+			return true;
+		}
+		for (int i = 0 ; i < descriptor.getElementsCount() ; i++) {
+			SerialDescriptor elementDescriptor = descriptor.getElementDescriptor(i);
+			if (!alreadyProcessed.contains(elementDescriptor.getSerialName()) && hasPolymorphism(elementDescriptor, alreadyProcessed)) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 }
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 3c30b2b0e0..8ee097d0ff 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
@@ -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.
@@ -17,12 +17,15 @@
 package org.springframework.http.codec.json;
 
 import java.lang.reflect.Type;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 
 import kotlinx.serialization.KSerializer;
 import kotlinx.serialization.SerializersKt;
 import kotlinx.serialization.descriptors.PolymorphicKind;
+import kotlinx.serialization.descriptors.SerialDescriptor;
 import kotlinx.serialization.json.Json;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
@@ -123,7 +126,7 @@ 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)) {
+			if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
 				throw new UnsupportedOperationException("Open polymorphic serialization is not supported yet");
 			}
 			serializerCache.put(type, serializer);
@@ -131,4 +134,18 @@ public class KotlinSerializationJsonEncoder extends AbstractEncoder<Object> {
 		return serializer;
 	}
 
+	private boolean hasPolymorphism(SerialDescriptor descriptor, Set<String> alreadyProcessed) {
+		alreadyProcessed.add(descriptor.getSerialName());
+		if (descriptor.getKind().equals(PolymorphicKind.OPEN.INSTANCE)) {
+			return true;
+		}
+		for (int i = 0 ; i < descriptor.getElementsCount() ; i++) {
+			SerialDescriptor elementDescriptor = descriptor.getElementDescriptor(i);
+			if (!alreadyProcessed.contains(elementDescriptor.getSerialName()) && hasPolymorphism(elementDescriptor, alreadyProcessed)) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 }
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 6025255e19..9b1f9752bd 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
@@ -20,12 +20,15 @@ import java.io.IOException;
 import java.lang.reflect.Type;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
+import java.util.HashSet;
 import java.util.Map;
+import java.util.Set;
 
 import kotlinx.serialization.KSerializer;
 import kotlinx.serialization.SerializationException;
 import kotlinx.serialization.SerializersKt;
 import kotlinx.serialization.descriptors.PolymorphicKind;
+import kotlinx.serialization.descriptors.SerialDescriptor;
 import kotlinx.serialization.json.Json;
 
 import org.springframework.core.GenericTypeResolver;
@@ -185,7 +188,7 @@ 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)) {
+			if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
 				throw new UnsupportedOperationException("Open polymorphic serialization is not supported yet");
 			}
 			serializerCache.put(type, serializer);
@@ -193,4 +196,18 @@ public class KotlinSerializationJsonHttpMessageConverter extends AbstractGeneric
 		return serializer;
 	}
 
+	private boolean hasPolymorphism(SerialDescriptor descriptor, Set<String> alreadyProcessed) {
+		alreadyProcessed.add(descriptor.getSerialName());
+		if (descriptor.getKind().equals(PolymorphicKind.OPEN.INSTANCE)) {
+			return true;
+		}
+		for (int i = 0 ; i < descriptor.getElementsCount() ; i++) {
+			SerialDescriptor elementDescriptor = descriptor.getElementDescriptor(i);
+			if (!alreadyProcessed.contains(elementDescriptor.getSerialName()) && hasPolymorphism(elementDescriptor, alreadyProcessed)) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 }
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 d333bd0a86..bd86de61cf 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
@@ -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.
@@ -56,6 +56,7 @@ class KotlinSerializationJsonDecoderTests : AbstractDecoderTests<KotlinSerializa
 				MediaType("application", "json", StandardCharsets.ISO_8859_1))).isTrue()
 
 		Assertions.assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Int::class.java), MediaType.APPLICATION_JSON)).isTrue()
+		Assertions.assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), MediaType.APPLICATION_JSON)).isFalse()
 		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()
@@ -101,6 +102,6 @@ class KotlinSerializationJsonDecoderTests : AbstractDecoderTests<KotlinSerializa
 
 
 	@Serializable
-	data class Pojo(val foo: String, val bar: String)
+	data class Pojo(val foo: String, val bar: String, val pojo: Pojo? = null)
 
 }
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 52aab80fe6..7d4b798075 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
@@ -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.
@@ -52,7 +52,8 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 				MediaType("application", "json", StandardCharsets.US_ASCII))).isTrue()
 
 		Assertions.assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Int::class.java), MediaType.APPLICATION_JSON)).isTrue()
-		Assertions.assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, KotlinSerializationJsonDecoderTests.Pojo::class.java), MediaType.APPLICATION_JSON)).isTrue()
+		Assertions.assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), MediaType.APPLICATION_JSON)).isFalse()
+		Assertions.assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Pojo::class.java), MediaType.APPLICATION_JSON)).isTrue()
 		Assertions.assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_JSON)).isTrue()
 		Assertions.assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_PDF)).isFalse()
 	}
@@ -97,6 +98,6 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 
 
 	@Serializable
-	data class Pojo(val foo: String, val bar: String)
+	data class Pojo(val foo: String, val bar: String, val pojo: Pojo? = null)
 
 }
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 3fe257c21f..5f950e6faf 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
@@ -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.
@@ -61,6 +61,7 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		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()
+		assertThat(converter.canRead(typeTokenOf<List<Ordered>>(), List::class.java, MediaType.APPLICATION_JSON)).isFalse()
 	}
 
 	@Test
@@ -315,7 +316,8 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 			val number: Int,
 			val string: String?,
 			val bool: Boolean,
-			val fraction: Float
+			val fraction: Float,
+			val serializableBean: SerializableBean? = null
 	)
 
 	data class NotSerializableBean(val string: String)