A convicted murderer on lifetime parole picked the wrong time to get physical on a crowded subway car Thursday, officials said.

Phillip Smalls, 48, of the Bronx, was spotted by a plainclothes team of transit cops on the southbound platform of the No. 4 Lexington Avenue line at Grand Central acting as if he was sizing up women to either grope or have sexual contact with them, said Chief of Transit Joseph Fox.

The team of three officers, including Jennifer Rosario, got on the train when Smalls did and observed him rubbing his groin against the back of a 22-year-old female passenger, Fox said. Cops moved in, separated Smalls from the woman and took them both off the train at the 14th Street station, he said.

Rosario then took a statement from her while Det. Rikki Rowdale and Sgt. Ivan Rodriguez took Smalls into custody, police said. Smalls, who was convicted of second-degree murder in 1988 and released from prison in 2013, faces charges of forceable touching and sexual abuse, Fox said.

“This is one of our best teams, a very active unit,” Fox said of the three officers.

Fox said he revitalized police efforts to go after subway sex abuse as a major quality-of-life offense after he attended a police conference on the subject in London in 2014.

“Not all men know that this happens, but far too many women do,” NYPD spokesman Martin Speechley said of subway groping.

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