He was reportedly an innocent bystander to the shooting, in which a 36-year-old man was targeted and seriously injured.
Eyewitnesses say the adult victim, a Serbian national, was being followed by the gunman, who fled the scene in a car after firing four shots.
Police are currently questioning witnesses and examining the crime scene for evidence. They believe the gunman is also Serbian.
Police spokesman Patrick Maierhofer said that there is some indication that the background to the shooting was a “previous dispute between the two men in Serbia”, but did not go into detail.
Shortly before 11.00am on Sunday the 36-year-old man was seen screaming for help on Pasettistrasse in Brigittenau.
He had just been buying bread from a local bakery but lost both his loaf and his slippers as he ran down the street, pursued by a man in a silver car with a Belgrade license plate, who fired several shots.
The 36-year-old was hit in the buttocks. The 13-year-old boy, who was cycling down the road, was hit in the abdomen and was taken to hospital in a critical condition.