A 27-year-old woman was followed into a Bed-Stuy apartment building and sexually assaulted early Sunday, police said.

The attack happened at 12:30 a.m. near Bedford Avenue and Willoughby Avenue in Brooklyn, according to the NYPD.

The suspect first slapped the woman’s rear end on the street before following her into the building and sexually assaulting her there, too, police said.

It is not clear if the woman knew the man.

He fled in an unknown direction, according to cops.

The victim was treated at a local hospital.