A man was fatally knifed outside a migrant shelter at Randalls Island on Saturday night and his killer was quickly arrested, police said.

A dispute near the front entrance of the migrant shelter turned lethal when the 26-year-old suspect plunged his blade into a 24-year-old man’s chest about 7:27 p.m., cops said.

Medics rushed the wounded man to Harlem Hospital, but he could not be saved, police said.

Cops promptly took the suspect into custody near the shelter.

It was not immediately clear what their dispute was about, police said.

The slaying was the second violent incident reported Saturday at a migrant facility.

Two cops suffered minor injuries to their faces and knees when a man assaulted them about 8:20 a.m. as they attempted to control a rowdy crowd outside a migrant center at the former St. Brigid School on E. Seventh St. near Avenue B in the East Village, cops said.

Angel Gutierrez Medal, 24, was arrested and charged with assaulting a police officer and resisting arrest.