An alleged subway groper was busted in Manhattan Wednesday afternoon in the assault of a woman who turned out to be an undercover cop.

The officer was groped on a downtown 5 train ­between Grand Central Station and Union Square at about 5:50 p.m., cops said.

Javier Monroy allegedly pressed his groin against the woman’s left hip.

At Union Square, she got off the train and told backup she had been the victim of a crime, and the ­39-year-old suspect was arrested, cops said.

Monroy was hit with charges of forcible touching and sexual abuse.