diff --git a/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/NewCookieHeaderDelegate.java b/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/NewCookieHeaderDelegate.java
index 77bff616144ab..686170816ac23 100644
--- a/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/NewCookieHeaderDelegate.java
+++ b/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/NewCookieHeaderDelegate.java
@@ -140,8 +140,21 @@ public String toString(Object value) {
             b.append(";HttpOnly");
         if (cookie.getSameSite() != null) {
             b.append(";SameSite=");
-            b.append(cookie.getSameSite());
+            appendCorrectCase(b, cookie.getSameSite());
         }
         return b.toString();
     }
+
+    private static void appendCorrectCase(final StringBuilder sb, final Enum<?> e) {
+        boolean first = true;
+        for (char c : e.name().toCharArray()) {
+            if (first) {
+                sb.append(c);
+                first = false;
+            } else {
+                sb.append(Character.toLowerCase(c));
+            }
+        }
+    }
+
 }
diff --git a/independent-projects/resteasy-reactive/server/vertx/src/test/java/org/jboss/resteasy/reactive/server/vertx/test/headers/CookiesTest.java b/independent-projects/resteasy-reactive/server/vertx/src/test/java/org/jboss/resteasy/reactive/server/vertx/test/headers/CookiesTest.java
index 3c6376fdc7713..0ed68ba267bbb 100644
--- a/independent-projects/resteasy-reactive/server/vertx/src/test/java/org/jboss/resteasy/reactive/server/vertx/test/headers/CookiesTest.java
+++ b/independent-projects/resteasy-reactive/server/vertx/src/test/java/org/jboss/resteasy/reactive/server/vertx/test/headers/CookiesTest.java
@@ -108,7 +108,7 @@ void testSameSite() {
                 .formParam("cookie", "greeting=\"hello\";SameSite=\"Lax\";")
                 .post("/cookies/set-cookie")
                 .then()
-                .cookie("greeting", detailedCookie().value("hello").sameSite("LAX"));
+                .cookie("greeting", detailedCookie().value("hello").sameSite("Lax"));
     }
 
     @Test
@@ -119,7 +119,7 @@ void testSameSiteWithoutColon() {
                 .formParam("cookie", "greeting=\"hello\";SameSite=\"None\"")
                 .post("/cookies/set-cookie")
                 .then()
-                .cookie("greeting", detailedCookie().value("hello").sameSite("NONE"));
+                .cookie("greeting", detailedCookie().value("hello").sameSite("None"));
     }
 
     @Test
@@ -130,7 +130,7 @@ void testSameSiteLowercase() {
                 .formParam("cookie", "greeting=\"hello\";samesite=\"Strict\"")
                 .post("/cookies/set-cookie")
                 .then()
-                .cookie("greeting", detailedCookie().value("hello").sameSite("STRICT"));
+                .cookie("greeting", detailedCookie().value("hello").sameSite("Strict"));
     }
 
     @Test