A college student was sexually assaulted and then robbed by a thug who followed her into her upper Manhattan home, police sources said.

The 21-year-old CCNY student was walking into her building at 1:30 p.m. yesterday when the man followed her inside.

As she was checking her mail, he said he had a gun and forced her into her apartment, where he assaulted her.

Minutes later, the suspect was caught on surveillance using the victim’s debit card.