The incident happened at the corner of 24th Avenue and Crescent Street in Astoria just after 9 p.m. on September 21.
Authorities say the man rode up on a bicycle and grabbed the 24-year-old woman's buttocks from behind before fleeing the scene.
The suspect is described as a Hispanic male, 20 to 30 years old, with a medium build. He is 5-foot-6 to 5-foot-10, with long curly Mohawk-style hair shaved on the sides. He was wearing a dark jacket with a red stripe on the sleeves and blue jeans. The bicycle was silver with a metal basket attached to the rear.
Anyone with information is asked to call Crime Stoppers at (800) 577-TIPS (8477).