The incident happened around 6:30 p.m. near the Atlantic Avenue stop on the No. 4 train on August 5.
Investigators say the man exposed himself and then rubbed against a 22-year-old female rider.
The suspect took off when the train stopped at Franklin Avenue in Crown Heights.
The man is described as a black male in his 50s, approximately 5-foot-10 and weighing about 250 pounds.
Anyone with information in regards to this forcible touching is asked to call the NYPD's Crime Stoppers Hotline at 800-577-TIPS.