diff --git a/extensions/resteasy-reactive/rest-client-reactive/deployment/src/test/java/io/quarkus/rest/client/reactive/headers/UserAgentTest.java b/extensions/resteasy-reactive/rest-client-reactive/deployment/src/test/java/io/quarkus/rest/client/reactive/headers/UserAgentTest.java
index 3e2c924ac9a05..d5341d80e3323 100644
--- a/extensions/resteasy-reactive/rest-client-reactive/deployment/src/test/java/io/quarkus/rest/client/reactive/headers/UserAgentTest.java
+++ b/extensions/resteasy-reactive/rest-client-reactive/deployment/src/test/java/io/quarkus/rest/client/reactive/headers/UserAgentTest.java
@@ -30,6 +30,12 @@ void testHeadersWithSubresource() {
         assertThat(client.call()).isEqualTo("Resteasy Reactive Client");
     }
 
+    @Test
+    void testHeaderOverride() {
+        Client client = RestClientBuilder.newBuilder().baseUri(baseUri).build(Client.class);
+        assertThat(client.callWithUserAgent("custom-agent")).isEqualTo("custom-agent");
+    }
+
     @Path("/")
     @ApplicationScoped
     public static class Resource {
@@ -44,6 +50,10 @@ public interface Client {
         @Path("/")
         @GET
         String call();
+
+        @Path("/")
+        @GET
+        String callWithUserAgent(@HeaderParam("User-AgenT") String userAgent);
     }
 
 }
diff --git a/independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/impl/InvocationBuilderImpl.java b/independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/impl/InvocationBuilderImpl.java
index 5126b4ab0c428..f9f25cf593cd0 100644
--- a/independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/impl/InvocationBuilderImpl.java
+++ b/independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/impl/InvocationBuilderImpl.java
@@ -51,9 +51,6 @@ public InvocationBuilderImpl(URI uri, ClientImpl restClient, HttpClient httpClie
         this.httpClient = httpClient;
         this.target = target;
         this.requestSpec = new RequestSpec(configuration);
-        if (restClient.getUserAgent() != null && !restClient.getUserAgent().isEmpty()) {
-            this.requestSpec.headers.header(HttpHeaders.USER_AGENT, restClient.getUserAgent());
-        }
         this.configuration = configuration;
         this.handlerChain = handlerChain;
         this.requestContext = requestContext;
@@ -67,11 +64,13 @@ public InvocationBuilderImpl(URI uri, ClientImpl restClient, HttpClient httpClie
 
     @Override
     public Invocation build(String method) {
+        setUserAgentIfNotSet();
         return new InvocationImpl(method, async(), null);
     }
 
     @Override
     public Invocation build(String method, Entity<?> entity) {
+        setUserAgentIfNotSet();
         return new InvocationImpl(method, async(), entity);
     }
 
@@ -97,6 +96,7 @@ public Invocation buildPut(Entity<?> entity) {
 
     @Override
     public AsyncInvokerImpl async() {
+        setUserAgentIfNotSet();
         return new AsyncInvokerImpl(restClient, httpClient, uri, requestSpec, configuration,
                 properties, handlerChain, requestContext);
     }
@@ -175,6 +175,7 @@ public CompletionStageRxInvoker rx() {
 
     @Override
     public <T extends RxInvoker> T rx(Class<T> clazz) {
+        setUserAgentIfNotSet();
         if (clazz == MultiInvoker.class) {
             return (T) new MultiInvoker(this);
         } else if (clazz == UniInvoker.class) {
@@ -189,6 +190,13 @@ public <T extends RxInvoker> T rx(Class<T> clazz) {
         return null;
     }
 
+    private void setUserAgentIfNotSet() {
+        if (!requestSpec.headers.getHeaders().containsKey(HttpHeaders.USER_AGENT)
+                && restClient.getUserAgent() != null && !restClient.getUserAgent().isEmpty()) {
+            this.requestSpec.headers.header(HttpHeaders.USER_AGENT, restClient.getUserAgent());
+        }
+    }
+
     @Override
     public Response get() {
         return unwrap(async().get());