Suffolk County police say the 23-year-old victim was walking near East Elm Street in Central Islip when the man came up and shot him.
The victim was able to waive down a passing motorist who drove him to the fire department. Firefighters stabilized the victim and then transported him to the hospital where he was listed in serious but stable condition.
Police ask anyone with information about the shooting to call Crime Stoppers at 1-800-220-TIPS. All calls will be kept confidential.