A man was fatally shot by his girlfriend’s old beau during a love triangle spat outside a Manhattan apartment complex, police said.

Darnell Brown, 23, and his girlfriend, 30, were outside a building on 1st Avenue near East 101st Street in East Harlem at about 7:45 p.m. Friday when the pair ran into the woman’s old flame, cops said.

Video shows ex-lover Sterling Stewart — clad in a mask — approaching the couple from behind and firing several rounds, law enforcement sources told The Post.

When police arrived, they found Brown with a gunshot wound to the lower back.

He was rushed to NYC Health + Hospitals/Metropolitan, where he was pronounced dead.

A second woman and an 8-year-old also witnessed the bloody attack, sources said.

Cops nabbed the 30-year-old Stewart a few blocks from the scene, where his old sweetheart identified him, saying she’d known Stewart for years but denying any romantic relationship, the sources said.

A semi-automatic 9mm gun was recovered at the scene.

Stewart was arrested and charged with murder, assault, criminal use of a firearm, and criminal possession of a weapon, officials said.

The spurned beau has at least eight past arrests, including charges for assault and robbery.