Police released a picture of the suspect that shows him smiling and waving.
The incident happened Friday morning on a southbound A train.
Police say the man exposed himself between the 175th Street station and the 34th Street station.
Anyone with information in regards to this incident is asked to call the NYPD's Crime Stoppers Hotline at 1-800-577-TIPS (8477) or for Spanish, 1-888-57-PISTA (74782).