A man slashed in the back in Union Square on Saturday chased his attacker back into the subway, police said.

The 29-year-old-victim had left the Union Square-14th Street station at 11:40 a.m. when a stranger came from behind and slashed him with an unidentified object, the NYPD said. The man then chased after his attacker, who boarded a 6 train that was headed northward, cops said.

The victim was taken to Mount Sinai Beth Israel Hospital to treat his injury.

Police released a photo of the male suspect, assumed to be 35- to 40-years old, who was wearing a camouflage shirt.