The couple, two men, were playing loud music in the train car, police said. Another rider didn't like it, yelled homophobic slurs, and then stabbed one man in the forearm and took his mobile phone, the NYPD said.
A video shot by a bystander shows the victim sitting on a platform at the E. 149th Street station in the Mott Haven section of the Bronx. The video shows the man with his arm raised in the air. Police said his boyfriend is seen standing by his side helping him.
The 31-year-old victim was taken to Lincoln Hospital to be treated for his injuries. Police said he was expected to recover.
Officers recovered the victim's phone at the scene. Police are investigating the attack as a possible hate crime.