diff --git a/compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/hotspot/GraalHotSpotVMConfig.java b/compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/hotspot/GraalHotSpotVMConfig.java
index ea338c321011..7beb49240367 100644
--- a/compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/hotspot/GraalHotSpotVMConfig.java
+++ b/compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/hotspot/GraalHotSpotVMConfig.java
@@ -35,6 +35,7 @@
 import jdk.graal.compiler.debug.Assertions;
 import jdk.graal.compiler.debug.GraalError;
 import jdk.graal.compiler.options.OptionValues;
+import jdk.graal.compiler.serviceprovider.JavaVersionUtil;
 import jdk.vm.ci.hotspot.HotSpotResolvedJavaMethod;
 import jdk.vm.ci.hotspot.HotSpotVMConfigStore;
 import jdk.vm.ci.meta.MetaAccessProvider;
@@ -87,6 +88,14 @@ public boolean useG1GC() {
         return gc == HotSpotGraalRuntime.HotSpotGC.G1;
     }
 
+    /*
+     * There's no direct way to test for the presence of the boolean flag ZGenerational since there
+     * aren't any notPresent values which are distinct from the possible values. Instead use true
+     * and false as the notPresent value and check if it returns different answers.
+     */
+    public final boolean isZGenerationalDefault = JavaVersionUtil.JAVA_SPEC > 21 &&
+                    (!access.getFlag("ZGenerational", Boolean.class, false).equals(access.getFlag("ZGenerational", Boolean.class, true)));
+
     public boolean useXGC() {
         return gc == HotSpotGraalRuntime.HotSpotGC.X;
     }
@@ -602,6 +611,13 @@ private long getZGCAddressField(String name) {
     public final int zPointerLoadShift = getConstant("ZPointerLoadShift", Integer.class, -1, osArch.equals("aarch64") && zgcSupport);
 
     private long getXGCAddressField(String name) {
+        if (JavaVersionUtil.JAVA_SPEC == 21 || isZGenerationalDefault) {
+            /*
+             * Graal does not support single gen ZGC in JDK 21 and it's no longer supported in 24
+             * once the ZGenerational global flag has been removed.
+             */
+            return 0;
+        }
         String realName = name;
         long address = 0;
         if (zgcSupport) {
diff --git a/compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/hotspot/HotSpotGraalRuntime.java b/compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/hotspot/HotSpotGraalRuntime.java
index 1c16b4c2a6dd..8319b8838d23 100644
--- a/compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/hotspot/HotSpotGraalRuntime.java
+++ b/compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/hotspot/HotSpotGraalRuntime.java
@@ -25,6 +25,7 @@
 package jdk.graal.compiler.hotspot;
 
 import static jdk.graal.compiler.core.common.GraalOptions.HotSpotPrintInlining;
+import static jdk.graal.compiler.serviceprovider.JavaVersionUtil.JAVA_SPEC;
 import static jdk.vm.ci.common.InitTimer.timer;
 import static jdk.vm.ci.hotspot.HotSpotJVMCIRuntime.runtime;
 
@@ -35,7 +36,6 @@
 import java.util.Map;
 import java.util.function.Predicate;
 
-import jdk.vm.ci.services.Services;
 import org.graalvm.collections.EconomicMap;
 import org.graalvm.collections.Equivalence;
 
@@ -75,6 +75,7 @@
 import jdk.vm.ci.meta.JavaKind;
 import jdk.vm.ci.meta.MetaAccessProvider;
 import jdk.vm.ci.runtime.JVMCIBackend;
+import jdk.vm.ci.services.Services;
 
 //JaCoCo Exclude
 
@@ -206,8 +207,8 @@ public enum HotSpotGC {
         Parallel("UseParallelGC"),
         G1("UseG1GC"),
         // non-generational ZGC
-        X(flagIsSet("UseZGC").and(flagIsNotSet("ZGenerational"))),
-        Z(flagIsSet("UseZGC").and(flagIsSet("ZGenerational"))),
+        X(JAVA_SPEC >= 24, true, flagIsSet("UseZGC").and(isZGenerational().negate())),
+        Z(JAVA_SPEC >= 24, true, flagIsSet("UseZGC").and(isZGenerational())),
         Epsilon(true, true, flagIsSet("UseEpsilonGC")),
 
         // Unsupported GCs
@@ -229,11 +230,20 @@ public enum HotSpotGC {
 
         private static Predicate<GraalHotSpotVMConfig> flagIsSet(String flag) {
             final boolean notPresent = false;
-            return config1 -> config1.getFlag(flag, Boolean.class, notPresent, true);
+            return config -> config.getFlag(flag, Boolean.class, notPresent, true);
         }
 
-        private static Predicate<GraalHotSpotVMConfig> flagIsNotSet(String flag) {
-            return flagIsSet(flag).negate();
+        private static Predicate<GraalHotSpotVMConfig> isZGenerational() {
+            return config -> {
+                if (JAVA_SPEC == 21) {
+                    return false;
+                }
+                if (config.isZGenerationalDefault) {
+                    return true;
+                } else {
+                    return config.getFlag("ZGenerational", Boolean.class);
+                }
+            };
         }
 
         /**
@@ -267,7 +277,7 @@ static HotSpotGC forName(int name, GraalHotSpotVMConfig config) {
                     // CollectedHeap::X is not defined in HotSpot. Query CollectedHeap::Z instead
                     // and the ZGenerational flag.
                     if (config.getConstant("CollectedHeap::Z", Integer.class, -1, gc.expectNamePresent) == name) {
-                        if (config.getFlag("ZGenerational", Boolean.class, false, true)) {
+                        if (isZGenerational().test(config)) {
                             return Z;
                         } else {
                             return X;