The incident happened around 8:30 p.m. at South Arlington Ridge Road and South Lang Street. According to police, a young adult male was walking on the sidewalk when a black male wearing a black hooded sweatshirt approached him and demanded money.
The victim was shot once in the leg and the suspect fled on foot with an undisclosed amount of cash, police said.
Police established a perimeter and brought in K-9 units and the Fairfax County Police helicopter to search for the suspect, but were unable to locate him. Both Gunston Middle School and nearby Oakridge Elementary School were hosting evening activities at the time and were locked down for a period after the incident.
The victim was transported to George Washington University hospital with what is described as a non-life-threatening injury, said Arlington County Police spokesman Dustin Sternbeck.
Some roads in the area are still blocked off as police continue to investigate the crime.
The last reported shooting in Arlington County occurred on May 29, 2012, outside of a hotel in Crystal City. A man suffered two non-life-threatening gunshot wounds during that incident.