Police nabbed the ex-con who allegedly groped a 4-year-old girl while she was sitting on a Manhattan-bound subway alongside her mother, authorities said.

Kwame Johnson, 42, was arrested Tuesday and charged with sex abuse, forcible touching, acting in a manner injurious to a child, menacing and two counts of harassment for last Thursday’s incident, police said.

The NYPD’s Special Victims Division made the arrest after an anonymous tipster provided police with information.

The child and her 33-year-old mom were seated in a southbound No. 4 train last week when Johnson boarded at Jerome Avenue and West 183rd Street in the Bronx. He sat down on the left side of the girl and placed his right hand under her buttocks, cops said.

When her mother saw what happened, she confronted Johnson — who then threatened to shoot her, police said.

Johnson is due to appear before a judge in the Bronx.

He previously served time in prison for a 1999 robbery conviction, and was released on parole in 2004.