diff --git a/spring-web/src/main/java/org/springframework/http/support/Netty4HeadersAdapter.java b/spring-web/src/main/java/org/springframework/http/support/Netty4HeadersAdapter.java
index 97e853d4b763..80d030261199 100644
--- a/spring-web/src/main/java/org/springframework/http/support/Netty4HeadersAdapter.java
+++ b/spring-web/src/main/java/org/springframework/http/support/Netty4HeadersAdapter.java
@@ -17,7 +17,9 @@
 package org.springframework.http.support;
 
 import java.util.AbstractSet;
+import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
@@ -51,6 +53,12 @@ public Netty4HeadersAdapter(HttpHeaders headers) {
 		this.headers = headers;
 	}
 
+	private void addAllIgnoreCase(String key, List<? extends String> values) {
+		List<String> listToUpdate = this.get(key);
+		listToUpdate = listToUpdate == null ? new ArrayList<>() : new ArrayList<>(listToUpdate);
+		listToUpdate.addAll(values);
+		this.headers.set(key, listToUpdate);
+	}
 
 	@Override
 	@Nullable
@@ -61,18 +69,18 @@ public String getFirst(String key) {
 	@Override
 	public void add(String key, @Nullable String value) {
 		if (value != null) {
-			this.headers.add(key, value);
+			addAllIgnoreCase(key, Collections.singletonList(value));
 		}
 	}
 
 	@Override
 	public void addAll(String key, List<? extends String> values) {
-		this.headers.add(key, values);
+		addAllIgnoreCase(key, values);
 	}
 
 	@Override
 	public void addAll(MultiValueMap<String, String> values) {
-		values.forEach(this.headers::add);
+		values.forEach(this::addAllIgnoreCase);
 	}
 
 	@Override
diff --git a/spring-web/src/main/java/org/springframework/http/support/Netty5HeadersAdapter.java b/spring-web/src/main/java/org/springframework/http/support/Netty5HeadersAdapter.java
index 26d7b7fd7c49..a633cc765499 100644
--- a/spring-web/src/main/java/org/springframework/http/support/Netty5HeadersAdapter.java
+++ b/spring-web/src/main/java/org/springframework/http/support/Netty5HeadersAdapter.java
@@ -53,6 +53,12 @@ public Netty5HeadersAdapter(HttpHeaders headers) {
 		this.headers = headers;
 	}
 
+	private void addAllIgnoreCase(String key, List<? extends String> values) {
+		List<String> listToUpdate = this.get(key);
+		listToUpdate = listToUpdate == null ? new ArrayList<>() : new ArrayList<>(listToUpdate);
+		listToUpdate.addAll(values);
+		this.headers.set(key, listToUpdate);
+	}
 
 	@Override
 	@Nullable
@@ -64,18 +70,18 @@ public String getFirst(String key) {
 	@Override
 	public void add(String key, @Nullable String value) {
 		if (value != null) {
-			this.headers.add(key, value);
+			addAllIgnoreCase(key, Collections.singletonList(value));
 		}
 	}
 
 	@Override
 	public void addAll(String key, List<? extends String> values) {
-		this.headers.add(key, values);
+		addAllIgnoreCase(key, values);
 	}
 
 	@Override
 	public void addAll(MultiValueMap<String, String> values) {
-		values.forEach(this.headers::add);
+		values.forEach(this::addAllIgnoreCase);
 	}
 
 	@Override
diff --git a/spring-web/src/test/java/org/springframework/http/server/reactive/HeadersAdaptersTests.java b/spring-web/src/test/java/org/springframework/http/server/reactive/HeadersAdaptersTests.java
index 9c7adf5451e6..068fcc06aafa 100644
--- a/spring-web/src/test/java/org/springframework/http/server/reactive/HeadersAdaptersTests.java
+++ b/spring-web/src/test/java/org/springframework/http/server/reactive/HeadersAdaptersTests.java
@@ -20,6 +20,7 @@
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Locale;
@@ -126,6 +127,42 @@ void shouldFailIfHeaderRemovedFromKeyset(MultiValueMap<String, String> headers)
 		assertThatThrownBy(names::remove).isInstanceOf(IllegalStateException.class);
 	}
 
+	@ParameterizedHeadersTest
+	void testAddHeadersOutput(MultiValueMap<String, String> headers) {
+		headers.add("TestHeader", "first");
+		headers.add("testHeader", "second");
+		MultiValueMap<String, String> multiValueMap =
+				CollectionUtils.toMultiValueMap(new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH));
+		headers.forEach(multiValueMap::addAll);
+		assertThat(multiValueMap.toString()).isEqualToIgnoringCase("{testheader=[first, second]}");
+	}
+
+	@ParameterizedHeadersTest
+	void testAddAllSingletonHeadersOutput(MultiValueMap<String, String> headers) {
+		headers.addAll("TestHeader", Collections.singletonList("first"));
+		headers.addAll("testHeader", Collections.singletonList("second"));
+		MultiValueMap<String, String> multiValueMap =
+				CollectionUtils.toMultiValueMap(new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH));
+		headers.forEach(multiValueMap::addAll);
+		assertThat(multiValueMap.toString()).isEqualToIgnoringCase("{testheader=[first, second]}");
+	}
+
+	@ParameterizedHeadersTest
+	void testAddAllMultipleHeadersOutput(MultiValueMap<String, String> headers) {
+		MultiValueMap<String, String> map1 =
+				CollectionUtils.toMultiValueMap(new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH));
+		map1.add("TestHeader", "first");
+		headers.addAll(map1);
+		MultiValueMap<String, String> map2 =
+				CollectionUtils.toMultiValueMap(new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH));
+		map2.add("testHeader", "second");
+		headers.addAll(map2);
+		MultiValueMap<String, String> multiValueMap =
+				CollectionUtils.toMultiValueMap(new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH));
+		headers.forEach(multiValueMap::addAll);
+		assertThat(multiValueMap.toString()).isEqualToIgnoringCase("{testheader=[first, second]}");
+	}
+
 	@Retention(RetentionPolicy.RUNTIME)
 	@Target(ElementType.METHOD)
 	@ParameterizedTest