Police were called by the ambulance service at about 2pm today, after the boy was found with a stab wound in Lovell Park, North Street.
The boy was taken to hospital by ambulance for emergency treatment and is described as being in a serious condition.
A police cordon is in place in the park and an investigation has begun.
Detectives are appealing for witnesses to the stabbing.
A spokesman for West Yorkshire Police said: "Anyone who witnessed it or who has any information that could assist the investigation is asked to contact Leeds District CID via 101 quoting crime reference 13180508838 or call the independent charity Crimestoppers anonymously on 0800 555 111."