A 13-year-old boy was fatally stabbed by another 13-year-old boy on an MTA bus Friday, cops and law-enforcement sources said.

The victim and his assailant — who both go to the same nearby middle school, IS 7 — were riding the S78 bus at Hylan Boulevard and Littlefield Avenue in Eltingville around 2:20 p.m. when they got into some kind of dispute, according to the NYPD and police sources. 

The assailant then stabbed his victim with a knife multiple times in the stomach, cops said.

The gravely wounded boy stumbled to the front of the bus holding his stomach and gasped to the driver that he was having trouble breathing, sources said.

He then walked to the back of the bus and collapsed to the floor, according to the sources.

The boy was taken to Staten Island University Hospital North, where he was pronounced dead less than an hour later, authorities said. 

The suspected assailant was taken into custody but not immediately charged. 

He had tossed the knife while fleeing the scene, sources said. 

The nature of the dispute between the teens was not immediately known.

Additional reporting by Tina Moore