A thug shouting homophobic slurs stabbed a gay straphanger traveling with his boyfriend and his sister on a No. 1 train Saturday evening, according to police.

Cops say the assailant got angry when he saw the victim, 28, with his partner, and argued with him as their northbound train pulled into 110th Street in Morningside Heights just after 7 p.m., cops said.

That’s when the attacker started yelling “you f—-t!” at the victim.

He then stabbed the victim in the left side of the chest and fled, authorities said.

Emergency responders rushed him to Mt. Sinai St. Luke’s Hospital, where he remains in serious but stable condition.

The stabber was still at large Saturday night.

Cops said the violence is being investigated as a hate crime.