A 53-year-old serial sexual predator was charged with grinding up against a woman on the No. 6 train after police caught him in the act, authorities said Wednesday.

Plainclothes officers spotted William Faulkner, who lists a homeless shelter on Ward Island as his home, on the southbound platform at the 86th Street station at about 1 p.m. Tuesday. He was walking very quickly, trying to catch up with a 30-year-old woman getting on the train, police said.

Officer Miguel Brand and Lt. Angela Morriss followed him onto the train. As the train pulled out from the 77th Street station, they watched Faulkner corner the woman and grind up against her, police said.

As the train traveled south, the woman tried to escape, ducking underneath his arm to get out. At 68th Street, police said she got onto another part of the train.

“You saw that?” the victim said to the officers when they approached her. At 59th Street, police were able to arrest Faulkner, cuffing him and taking him off the train.

Faulkner was hit with two charges of sexual abuse, police said.

This wasn’t his first run-in with police. Faulkner has several prior sexual abuse-related arrests, including three convictions for offenses in 2009, 2010, and 2012.

Faulkner was even arrested as early as 1983 for public lewdness, a law enforcement official said.

Faulkner was awaiting arraignment Wednesday. Attorney information for him was not immediately available.