The attack happened at 12:35 p.m. on July 22 inside a building at Bedford and Flatbush avenues in Flatbush.
Police say the man followed the girl into a stairwell and then sexually assaulted her.
He ran away towards Caton Avenue.
The suspect is described as a black male in his 30s, with dark skin, about 5'11" tall with a medium build.
He was last seen wearing a gray baseball hat, gray sweatpants and a blue shirt.
Anyone with information in regards to this incident is asked to call Crime stoppers at 1-800-577-TIPS (8477).
You can also submit your tips by logging onto the Crime stoppers website at www.NYPDCRIMESTOPPERS.com or by texting your tips to 274637 (CRIMES) and then entering TIP577.
All calls are strictly confidential.