diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 3bda2a7a..bc39b9fb 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -11,6 +11,12 @@ jobs:
     strategy:
       matrix:
         java: [ 11, 17 ]
+        # Custom JDK 21 configuration
+        include:
+          - java: 21
+            # Disable Enforcer check which (intentionally) prevents using JDK 21 for building
+            # Exclude 'shrinker-test' module because R8 does not support JDK 21 yet
+            extra-mvn-args: -Denforcer.fail=false --projects '!:shrinker-test'
     runs-on: ubuntu-latest
 
     steps:
@@ -23,7 +29,7 @@ jobs:
           cache: 'maven'
       - name: Build with Maven
         # This also runs javadoc:jar to detect any issues with the Javadoc generated during release
-        run: mvn --batch-mode --no-transfer-progress verify javadoc:jar
+        run: mvn --batch-mode --no-transfer-progress verify javadoc:jar ${{ matrix.extra-mvn-args || '' }}
 
   native-image-test:
     name: "GraalVM Native Image test"
diff --git a/gson/pom.xml b/gson/pom.xml
index 527800d3..1799ef03 100644
--- a/gson/pom.xml
+++ b/gson/pom.xml
@@ -191,6 +191,19 @@
             </goals>
           </execution>
         </executions>
+        <!-- Upgrades ProGuard to version newer than the one included by plugin by default -->
+        <dependencies>
+          <dependency>
+            <groupId>com.guardsquare</groupId>
+            <artifactId>proguard-base</artifactId>
+            <version>7.4.1</version>
+          </dependency>
+          <dependency>
+            <groupId>com.guardsquare</groupId>
+            <artifactId>proguard-core</artifactId>
+            <version>9.1.0</version>
+          </dependency>
+        </dependencies>
         <configuration>
           <obfuscate>true</obfuscate>
           <injar>test-classes-obfuscated-injar</injar>
diff --git a/gson/src/test/java/com/google/gson/functional/ReflectionAccessTest.java b/gson/src/test/java/com/google/gson/functional/ReflectionAccessTest.java
index 165d671b..f7b2e335 100644
--- a/gson/src/test/java/com/google/gson/functional/ReflectionAccessTest.java
+++ b/gson/src/test/java/com/google/gson/functional/ReflectionAccessTest.java
@@ -18,6 +18,7 @@ package com.google.gson.functional;
 
 import static com.google.common.truth.Truth.assertThat;
 import static org.junit.Assert.fail;
+import static org.junit.Assume.assumeTrue;
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
@@ -52,6 +53,9 @@ public class ReflectionAccessTest {
   @SuppressWarnings("removal") // java.lang.SecurityManager deprecation in Java 17
   @Test
   public void testRestrictiveSecurityManager() throws Exception {
+    // Skip for newer Java versions where `System.setSecurityManager` is unsupported
+    assumeTrue(Runtime.version().feature() <= 17);
+
     // Must use separate class loader, otherwise permission is not checked, see
     // Class.getDeclaredFields()
     Class<?> clazz = loadClassWithDifferentClassLoader(ClassWithPrivateMembers.class);
diff --git a/pom.xml b/pom.xml
index 3e89ab58..0a9a82cd 100644
--- a/pom.xml
+++ b/pom.xml
@@ -463,6 +463,20 @@
       </build>
     </profile>
 
+    <!-- Slightly adjust build to support building with JDK >= 21
+      However, by default this will intentionally be blocked by the Maven Enforcer Plugin defined above
+      and must be explicitly circumvented to avoid building non-release Gson artifacts by accident -->
+    <profile>
+      <id>jdk21+</id>
+      <activation>
+        <jdk>[21,)</jdk>
+      </activation>
+      <properties>
+        <!-- JDK 21 does not support Java 7 as release, must use at least Java 8 -->
+        <maven.compiler.release>8</maven.compiler.release>
+      </properties>
+    </profile>
+
     <!-- Profile defining additional plugins to be executed for release -->
     <profile>
       <id>release</id>
diff --git a/proto/pom.xml b/proto/pom.xml
index 75fffa9e..9d1cec26 100644
--- a/proto/pom.xml
+++ b/proto/pom.xml
@@ -31,6 +31,8 @@
     <!-- Make the build reproducible, see root `pom.xml` -->
     <!-- This is duplicated here because that is recommended by `artifact:check-buildplan` -->
     <project.build.outputTimestamp>2023-01-01T00:00:00Z</project.build.outputTimestamp>
+
+    <protobufVersion>3.25.1</protobufVersion>
   </properties>
 
   <licenses>
@@ -50,7 +52,7 @@
     <dependency>
       <groupId>com.google.protobuf</groupId>
       <artifactId>protobuf-java</artifactId>
-      <version>4.0.0-rc-2</version>
+      <version>${protobufVersion}</version>
     </dependency>
 
     <dependency>
@@ -89,7 +91,7 @@
         <artifactId>protobuf-maven-plugin</artifactId>
         <version>0.6.1</version>
         <configuration>
-          <protocArtifact>com.google.protobuf:protoc:3.17.3:exe:${os.detected.classifier}</protocArtifact>
+          <protocArtifact>com.google.protobuf:protoc:${protobufVersion}:exe:${os.detected.classifier}</protocArtifact>
         </configuration>
         <executions>
           <execution>
diff --git a/shrinker-test/pom.xml b/shrinker-test/pom.xml
index 468d88e6..c4bd4f26 100644
--- a/shrinker-test/pom.xml
+++ b/shrinker-test/pom.xml
@@ -104,6 +104,19 @@
             </goals>
           </execution>
         </executions>
+        <!-- Upgrades ProGuard to version newer than the one included by plugin by default -->
+        <dependencies>
+          <dependency>
+            <groupId>com.guardsquare</groupId>
+            <artifactId>proguard-base</artifactId>
+            <version>7.4.1</version>
+          </dependency>
+          <dependency>
+            <groupId>com.guardsquare</groupId>
+            <artifactId>proguard-core</artifactId>
+            <version>9.1.0</version>
+          </dependency>
+        </dependencies>
         <configuration>
           <obfuscate>true</obfuscate>
           <proguardInclude>${project.basedir}/proguard.pro</proguardInclude>
@@ -205,6 +218,7 @@
               but it appears that can be ignored -->
             <groupId>com.android.tools</groupId>
             <artifactId>r8</artifactId>
+            <!-- TODO: When updating to a version which supports JDK 21, remove 'shrinker-test' exclusion from build.yml workflow -->
             <version>8.1.72</version>
           </dependency>
         </dependencies>