Paterson police say city resident Kenneth Santilla and another 13-year-old boy were walking along a street when the attack occurred Friday afternoon at an undisclosed location in the Hillcrest section of Paterson.
The boys then ran off in different directions.
Santilla was found dead late Friday night, shortly after he was reported missing. An autopsy is pending.
The other teen and the dog's owner both suffered bite wounds. The other boy's name was not released.
Authorities have said 50-year-old dog owner Paul Clarke stabbed the animal "quite a few times" during the attack.
The dog was euthanized, and its remains will be tested for rabies.
No charges have been filed.