The suspect, who can be seen in photos released Tuesday, sat next to a 26-year-old woman aboard the southbound train and stuck his phone under her skirt to take photos about 10:10 p.m. on Aug. 31, NYPD officials said.
The man left the train at Kings Highway/East 16th Street subway station and hadn't been arrested as of Wednesday, police said.
He's about 30, 5 feet 6 and 180 pounds, police said.
He was last seen wearing a shirt with "Instagram" across the chest and camouflage shorts, police said.
Anyone with information should contact Crime Stoppers at (800) 577-TIPS (8477).