diff --git a/sdks/java/extensions/kryo/src/main/java/org/apache/beam/sdk/extensions/kryo/KryoCoder.java b/sdks/java/extensions/kryo/src/main/java/org/apache/beam/sdk/extensions/kryo/KryoCoder.java
index 8f6660cf5b78..e89a61f43ab8 100644
--- a/sdks/java/extensions/kryo/src/main/java/org/apache/beam/sdk/extensions/kryo/KryoCoder.java
+++ b/sdks/java/extensions/kryo/src/main/java/org/apache/beam/sdk/extensions/kryo/KryoCoder.java
@@ -41,6 +41,13 @@
  * Coder using Kryo as (de)serialization mechanism. See {@link KryoCoderProvider} to get more
  * details about usage.
  *
+ * <p><b>Warning:</b> Kryo deserialization (especially with the default {@code
+ * kryoRegistrationRequired=false} setting in {@link KryoOptions}) can instantiate arbitrary classes
+ * and is unsafe when applied to untrusted data. Do not use {@link KryoCoder} to deserialize
+ * untrusted external input. To restrict deserialization to an explicit set of allowed classes,
+ * enable {@link KryoOptions#setKryoRegistrationRequired(boolean)} and register classes via {@link
+ * KryoRegistrar}.
+ *
  * @param <T> type of element coder can handle
  */
 public class KryoCoder<T> extends CustomCoder<T> {