The shooting happened around 9:24 a.m. at Harbor Food Mart at 3830 Guthrie Street in East Chicago, police said.
The victim was shot in the head, police said. He was taken to a local hospital, where he was later pronounced dead.
The Lake County Coroner's Office in Indiana said the family of Dereon Fields, a 17-year-old East Chicago boy, identified him at the hospital.
East Chicago police investigators said the suspect fled the scene on foot and was reportedly wearing a black ski mask.
No arrests have been made as police and the Lake County Prosecutor's Homicide Task Force is investigating.