A sicko followed a 12-year-old girl into her Manhattan apartment building and attempted to sexually assault her, police said yesterday.

She was walking up the stairs of the building on Hester Street on the Lower East Side at 6 p.m. Monday when she heard someone outside.

She let him in for reasons that were not immediately clear, and he followed her to the third floor and tried to attack her.

But before he could harm her, a noise spooked him and he fled.

Cops said the girl did not know the perv.