The incident happened just before 7 p.m. last Thursday inside Washington Park, located on 3rd Street between 4th and 5th avenues, in Park Slope.
Authorities say the suspect approached the 12-year-old female victim and grabbed her buttocks before fleeing the scene.
He is described only as an approximately 30-year-old male.
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).