The incident happened on S. Buchanan Street, just off Columbia Pike, around 6 p.m.
Police say the man fled on foot when confronted.
More from an Arlington County Police Department crime report:
SEX OFFENSE, 2017-10130223, 900 block of S. Buchanan Street. At approximately 6:03 p.m. on October 13, police were dispatched to the report of a male masturbating in public. Upon arrival, it was determined that an unknown male suspect was observed masturbating behind a tree while watching children playing. The suspect fled on foot when approached by an adult in the area. The suspect is described as a dark[-skinned] male in his 40’s wearing a navy blue hoodie, light washed jeans and a brown hat. The investigation is ongoing.