diff --git a/spring-test/src/main/java/org/springframework/test/web/reactive/server/UserWebTestClientConfigurer.java b/spring-test/src/main/java/org/springframework/test/web/reactive/server/UserWebTestClientConfigurer.java
new file mode 100644
index 00000000000..97e43d5a3a7
--- /dev/null
+++ b/spring-test/src/main/java/org/springframework/test/web/reactive/server/UserWebTestClientConfigurer.java
@@ -0,0 +1,110 @@
+/*
+ * Copyright 2002-present 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.test.web.reactive.server;
+
+import java.security.cert.X509Certificate;
+
+import org.jspecify.annotations.Nullable;
+import reactor.core.publisher.Mono;
+
+import org.springframework.http.client.reactive.ClientHttpConnector;
+import org.springframework.http.server.reactive.SslInfo;
+import org.springframework.util.Assert;
+import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.server.WebFilter;
+import org.springframework.web.server.WebFilterChain;
+import org.springframework.web.server.adapter.WebHttpHandlerBuilder;
+
+/**
+ * {@link WebTestClientConfigurer} that modifies WebFlux mock server requests
+ * by setting their {@link SslInfo}.
+ *
+ * <p>You can apply the configurer to a {@link WebTestClient.Builder}:
+ *
+ * <pre class="code">
+ * WebTestClient client = webTestClientBuilder
+ *         .apply(UserWebTestClientConfigurer.x509(certificate))
+ *         .build();
+ * </pre>
+ *
+ * <p>Or mutate an already built {@link WebTestClient}:
+ *
+ * <pre class="code">
+ * WebTestClient newClient =
+ *         client.mutateWith(UserWebTestClientConfigurer.x509(certificate));
+ * </pre>
+ *
+ * <p><strong>Note:</strong> This configurer is applicable only to WebFlux mock
+ * server setup. For a {@code WebTestClient.Builder} with a liver server setup,
+ * or a non-WebFlux, mock server, {@link IllegalStateException} is raised.
+ *
+ * <p>For tests with a MockMvc server, refer to a similar facility to set the
+ * user identity per request through Spring Security's
+ * {@code SecurityMockMvcRequestPostProcessors}.
+ *
+ * @author Rossen Stoyanchev
+ * @since 7.0
+ */
+public final class UserWebTestClientConfigurer implements WebTestClientConfigurer {
+
+	private final SslInfo info;
+
+
+	private UserWebTestClientConfigurer(SslInfo info) {
+		this.info = info;
+	}
+
+
+	@Override
+	public void afterConfigurerAdded(
+			WebTestClient.Builder builder, @Nullable WebHttpHandlerBuilder httpHandlerBuilder,
+			@Nullable ClientHttpConnector connector) {
+
+		Assert.state(httpHandlerBuilder != null, "This configurer is applicable only to a mock WebFlux server");
+		httpHandlerBuilder.filters(filters -> filters.add(0, new UserWebFilter()));
+	}
+
+
+	/**
+	 * Create a configurer with the given {@link X509Certificate X509 certificate(s)}.
+	 */
+	public static UserWebTestClientConfigurer x509(X509Certificate... certificates) {
+		return sslInfo(SslInfo.from("1", certificates));
+	}
+
+	/**
+	 * Create a configurer with the given {@link SslInfo}.
+	 */
+	public static UserWebTestClientConfigurer sslInfo(SslInfo info) {
+		return new UserWebTestClientConfigurer(info);
+	}
+
+
+	private final class UserWebFilter implements WebFilter {
+
+		@Override
+		public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
+
+			exchange = exchange.mutate()
+					.request(builder -> builder.sslInfo(UserWebTestClientConfigurer.this.info))
+					.build();
+
+			return chain.filter(exchange);
+		}
+	}
+
+}
diff --git a/spring-test/src/test/java/org/springframework/test/web/reactive/server/MockServerSpecTests.java b/spring-test/src/test/java/org/springframework/test/web/reactive/server/MockServerSpecTests.java
index dc1a0ce5868..faffa388906 100644
--- a/spring-test/src/test/java/org/springframework/test/web/reactive/server/MockServerSpecTests.java
+++ b/spring-test/src/test/java/org/springframework/test/web/reactive/server/MockServerSpecTests.java
@@ -22,6 +22,7 @@ import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Mono;
 
+import org.springframework.cglib.core.internal.Function;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
 import org.springframework.http.server.reactive.SslInfo;
@@ -94,9 +95,25 @@ public class MockServerSpecTests {
 
 	@Test
 	void sslInfo() {
+		testSslInfo(info -> this.serverSpec.sslInfo(info).build());
+	}
+
+	@Test
+	void sslInfoViaWebTestClientConfigurer() {
+		testSslInfo(info -> this.serverSpec.configureClient().apply(UserWebTestClientConfigurer.sslInfo(info)).build());
+	}
+
+	@Test
+	void sslInfoViaMutate() {
+		testSslInfo(info -> this.serverSpec.build().mutateWith(UserWebTestClientConfigurer.sslInfo(info)));
+	}
+
+	private void testSslInfo(Function<SslInfo, WebTestClient> function) {
 		SslInfo info = SslInfo.from("123");
-		this.serverSpec.sslInfo(info).build().get().uri("/").exchange().expectStatus().isOk();
-		assertThat(this.serverSpec.getSavedSslInfo()).isSameAs(info);
+		function.apply(info).get().uri("/").exchange().expectStatus().isOk();
+
+		SslInfo actual = this.serverSpec.getSavedSslInfo();
+		assertThat(actual).isSameAs(info);
 	}