A Bronx woman was arrested for her role in a brutal Upper West Side attack that left a woman with a fractured face, police said.

Trinity Velez, 18, of the Bronx was one of a gang of four that approached a 57-year-old woman from behind on Amsterdam Ave near 68th St. on Oct 17, physically assaulting her and then making a grab for her purse, cops said.

Surveillance footage of the incident shows one of the attackers smacking the victim on the side of the head.

A second suspect then drags the woman off-camera, the video shows.

Seconds later, two others attempt to grab the woman’s bag, then run off.

The woman was taken to St, Luke’s Roosevelt Hospital and treated for fractured bones in her face, authorities said.

Velez, who was busted Thursday for her part in the group assault, was charged with robbery, police said.

Investigators are still searching for three remaining suspects.