An off-duty MTA employee was arrested Thursday by a sharp-eyed transit officer who saw him press up against a woman riding a train at Union Square during rush hour, the NYPD said.

David Grant, a 46-year-old bus driver, was charged with forcible touching and sexual abuse after he was arrested on a Lexington Avenue line train around 9:30 a.m., according to a police spokesman.

A transit officer saw Grant allegedly press his body against the 36-year-old woman and then arrested him. The NYPD spokesman did not have information on whether the officer was wearing plainclothes or in uniform.

Grant, who worked out of the Manhattanville Depot in Harlem, had been with the MTA since July 2012, according to MTA spokesman Kevin Ortiz.

Grant was booted from operating a bus and will face disciplinary action pending the outcome of his case, Ortiz said.