A woman was sexually assaulted by a man who followed her into a building in Park Slope, police said.

The suspect followed the 29-year-old victim into a residential building in the vicinity of 3rd Avenue and 8th Street at around 11 p.m. Wednesday, cops said.

Once inside, the predator approached her from behind, reached under her skirt and groped her, the NYPD said.

The assailant is described as a man in his 30s, approximately 5-foot-6 inches tall, with a medium complexion, medium build and short black hair, police said. He was last seen wearing a blue hooded jacket, a dark colored backpack, black jeans and white sneakers.

The attacker ran off. The victim was treated at Kings County Hospital and released. The NYPD Special Victims Unit is investigating.

Police on Saturday released a surveillance video and photo of the suspect, taken before the incident in front of 239 9th St., cops said.

Anyone with information in regard to this incident is asked to call the NYPD’s Crime Stoppers Hotline at 1-800-577-TIPS (8477). The public can also submit their tips by logging onto the Crime Stoppers website at https://crimestoppers.nypdonline.org/, or on Twitter @NYPDTips.