The 36-year-old woman was taken to Robert Wood Johnson hospital in New Brunswick, where she is in stable condition.
Authorities say the victim was walking on Harper Street around 5:30 p.m. Sunday when the suspect approached her, forced her into the basement of a nearby apartment building at gunpoint and repeatedly punched her before sexually assaulting her and fleeing.
Police were alerted when a neighbor called the police after the victim ran to his or her home following the attack.
The suspect is described as a black male in his early 20s, about 5-foot-7 with a medium build and short hair. He was wearing a red T-shirt, black jacket and gray sweatpants.
Anyone with information is asked to call Detective McGraw of the Highland Park Police Department at 732-572-3800 or Detective Bitterman of the Middlesex County Prosecutor's office at 732-745-4401.