The 4-year-old from Rochester Hills was in his mother's car while she was loading groceries in the back when he found her gun in the glove compartment and pulled the trigger, police say.
"He pulled the trigger on the gun, shooting himself in the upper right area, (it) exited out the right thigh," Troy, Michigan Police Capt. Bob Redmond told Fox2Detroit.
The boy was conscious when he was taken away in an ambulance, according to witnesses. He is expected to recover.
The weapon was a 9 MM handgun and the boy's mother held a concealed pistol license.
However, Redmond was clear that the mother should shoulder some of the blame.
"The purpose of a CPL is so you can carry a handgun, then carry it. Don't store it in a car," he said.
The mother could face charges of careless, reckless, or negligent use of a firearm though police said pressing charges would be up to the prosecutor's office.