He is charged with murder and one count of armed robbery.
"The arrestee is a juvenile and therefore, his name and information will not be released," Sgt. Greg Lyon, an Atlanta Police Department spokesman, wrote in an e-mail.
"I want to stress that since this is an active and ongoing investigation, we will not be able to release additional information at this time."
Arnold, 21, was shot in the head in what police say is a robbery on the trail between 200 blocks of Warren and Rogers streets in the city's northeastern quadrant..
One of Arnold's friends suffered a gunshot wound and a third person was unharmed.
The three were walking along the bike trail when the two men approached them around 5:30 p.m.
The two shooting suspects got into a physical altercation with Arnold and the other man before the victims were shot.
Arnold's death is the 15th homicide in APD's Zone 6 — which includes Kirkwood, East Atlanta, Grant Park, Cabbagetown and Candler Park, among other neighborhoods.
Neighborhood Planning Unit-O is holding a vigil Saturday, Dec. 28 at 5 p.m. to show support for Arnold's family at 241 Warren St.
Participants are also invited to wear or carry a large "X" in commemoration of Mr. Arnold.