The victim was aboard the BX6 bus near Hunts Point and Lafayette avenues when three men approached him from behind about 10 p.m. and snatched his iPhone from his hand, police said.
The man, whom police did not identify, followed the suspects as they exited the bus, and one of the men shot him in the right leg.
The victim was taken to Lincoln Hospital, police said. He was in stable condition.
All three suspects were photographed by a surveillance camera in the bus, NYPD officials said.
The men were described by police as being in their 20s and having thin builds. One man wore a black T-shirt, another had a black, hooded sweatshirt, and the third wore a gray, hooded sweatshirt. They all wore dark pants, cops added.
Anyone with information about the incident is asked to call the NYPD's Crime Stoppers hotline at 800-577-TIPS (8477). Members of the public can also submit tips by logging onto the Crime Stoppers website at www.nypdcrimestoppers.com, or by texting tips to CRIMES (274637), then entering TIP577.