The subway rider who was threatened by a knife-wielding dad on a train in Manhattan told The Post that he reacted “completely by instinct” when the man struck his own child.

The straphanger, who works at the Permanent Mission of Brazil near the United Nations, said the chaos started when the angry dad hit his child and became incensed when the victim looked at him.

“We made eye contact. He said, ‘Why you looking at me?'” recalled the subway rider who only gave his name as Robson.

“I’m looking at you because you just hit your son. I think it’s not necessary,” Robson told him.

Robson, who was on a northbound E train with his wife and child on June 24, said that statement caused the dad to go on his tirade, which was caught on video.

“He pulled a knife, ‘Come over here. I’m gonna kill you. I’m gonna kill you,” Robson recalled.

As Robson tried to console his crying daughter, the maniac dad taunted her: “It will be OK. Your daddy will get killed on this train.”

The knife-wielding man finally exited the train and Robson — who usually ignores strange subway behavior — said he could not let the child abuse go unnoticed.

“I ride the subway every single day. I see a lot of crazy people looking for fights. If someone approaches you, you put your head down and walk away,” he said.

“I think I reacted completely by instinct. When I saw him hitting a child, I just opened my mouth.”

Cops were looking for the knife-wielding maniac on Monday.