Extending Mike Lively’s answer with a code sample & referring to Oracle too.
I had this problem with the OracleDialect (Oracle10gDialect). Adding an annotation @Type to the UUID field fixed it for me.
@Id
@Type(type="uuid-char")
private UUID id;
Note: also used a TwoWayStringBridge on this field, using the @FieldBridge annotation.
Note: type=”uuid-binary” did not work; got the same, unknown type error.