Just before 2 p.m. Sunday, police say a car hit the boys while they were walking on the side of David Terry Road in Manorville.
The scouts from Troop 161 were headed for a hike and were with adults when the crash happened. In all, five boys ranging in age from 12 to 16 were injured.
Andrew McMorris, 12, died of his injuries. He was in 7th grade at Albert Prodell Middle School.
The Suffolk County Council, Boy Scouts of America confirmed McMorris's death on Monday.
Thomas Lake, 15, was airlifted to Stony Brook University Hospital where he is being treated for serious injuries.
Denis Lane, 16, Matthew Yakaboski, 15, and Kaden Lynch, 15, suffered non-life threatening injuries and were being treated at Peconic Bay Medical Center.
Police arrested 59-year-old Thomas Murphy of Holbrook and charged him with driving while intoxicated.