diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/DefaultServerHttpRequestBuilder.java b/spring-web/src/main/java/org/springframework/http/server/reactive/DefaultServerHttpRequestBuilder.java
index 847669e0eb..d4398d7609 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/DefaultServerHttpRequestBuilder.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/DefaultServerHttpRequestBuilder.java
@@ -38,6 +38,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Rossen Stoyanchev
  * @author Sebastien Deleuze
+ * @author Brian Clozel
  * @since 5.0
  */
 class DefaultServerHttpRequestBuilder implements ServerHttpRequest.Builder {
@@ -131,7 +132,7 @@ class DefaultServerHttpRequestBuilder implements ServerHttpRequest.Builder {
 	@Override
 	public ServerHttpRequest build() {
 		return new MutatedServerHttpRequest(getUriToUse(), this.contextPath,
-				this.httpMethodValue, this.sslInfo, this.remoteAddress, this.body, this.originalRequest);
+				this.httpMethodValue, this.sslInfo, this.remoteAddress, this.headers, this.body, this.originalRequest);
 	}
 
 	private URI getUriToUse() {
@@ -190,9 +191,9 @@ class DefaultServerHttpRequestBuilder implements ServerHttpRequest.Builder {
 
 		public MutatedServerHttpRequest(URI uri, @Nullable String contextPath,
 				String methodValue, @Nullable SslInfo sslInfo, @Nullable InetSocketAddress remoteAddress,
-				Flux<DataBuffer> body, ServerHttpRequest originalRequest) {
+				HttpHeaders headers, Flux<DataBuffer> body, ServerHttpRequest originalRequest) {
 
-			super(uri, contextPath, originalRequest.getHeaders());
+			super(uri, contextPath, headers);
 			this.methodValue = methodValue;
 			this.remoteAddress = (remoteAddress != null ? remoteAddress : originalRequest.getRemoteAddress());
 			this.sslInfo = (sslInfo != null ? sslInfo : originalRequest.getSslInfo());
diff --git a/spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpRequestTests.java b/spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpRequestTests.java
index d9ca717ca6..7eca5ed6e1 100644
--- a/spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpRequestTests.java
+++ b/spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpRequestTests.java
@@ -30,6 +30,7 @@ import org.junit.jupiter.api.Test;
 
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
 import org.springframework.http.HttpMethod;
+import org.springframework.http.MediaType;
 import org.springframework.util.MultiValueMap;
 import org.springframework.web.testfixture.servlet.DelegatingServletInputStream;
 import org.springframework.web.testfixture.servlet.MockAsyncContext;
@@ -46,6 +47,7 @@ import static org.mockito.Mockito.mock;
  *
  * @author Rossen Stoyanchev
  * @author Sam Brannen
+ * @author Brian Clozel
  */
 public class ServerHttpRequestTests {
 
@@ -166,6 +168,18 @@ public class ServerHttpRequestTests {
 		assertThat(request.getHeaders().get(headerName)).containsExactly(headerValue3);
 	}
 
+	@Test // gh-26615
+	void mutateContentTypeHeaderValue() throws Exception {
+		ServerHttpRequest request = createRequest("/path").mutate()
+				.headers(headers -> headers.setContentType(MediaType.APPLICATION_JSON)).build();
+
+		assertThat(request.getHeaders().getContentType()).isEqualTo(MediaType.APPLICATION_JSON);
+
+		ServerHttpRequest mutated = request.mutate()
+				.headers(headers -> headers.setContentType(MediaType.APPLICATION_CBOR)).build();
+		assertThat(mutated.getHeaders().getContentType()).isEqualTo(MediaType.APPLICATION_CBOR);
+	}
+
 	@Test
 	void mutateWithExistingContextPath() throws Exception {
 		ServerHttpRequest request = createRequest("/context/path", "/context");