A 13-year-old boy was fatally stabbed by another 13-year-old boy on an MTA bus Friday — and the suspected killer was nabbed by a retired cop who happened to be in the area, police and sources said.

Syles Ular 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. 

“Witnesses state there was an argument on the bus, a knife is produced, and gang signs were being shown back and forth,” NYPD Chief of Patrol John Chell told reporters at a press conference at police headquarters Friday night.

The suspect then stabbed Ular with a knife multiple times in the chest, 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 young suspect fled the bus and ran about three blocks, tossing the murder weapon in the process, according to police.

A retired NYPD sergeant who happened to be in the area began following the boy, Chell said.

Another witness pointed out the assailant and told the retired cop, “This person just stabbed somebody,” the chief said. 

 The retired cop then arrested the teenager, who was taken into police custody.

“We believe this motive possibly is gang-related,” Chell said.

Demetrius Crichlow, senior vice president of the NYC Transit Department, said he responded to the scene and met with the MTA bus driver.

“He was visibly broken up about what transpired,” he said. The driver “notified communications, contacted the police rapidly and did a fantastic job of trying to maintain calm on the bus.”

Additional reporting by Tina Moore