A three-year-old boy has shot his mother dead in a supermarket car park south of Chicago.

The pair were in a vehicle outside a Food 4 Less store in the village of Dolton when the child somehow found the gun and fired it, police said.

His mum was pronounced dead at hospital.

His father is in custody after indicating he owned the gun, authorities said.

There are no charges as yet.

Dolton trustee Andrew Holmes visited the supermarket to hand out gun locks and speak to shoppers about firearm safety.

“All it takes is a second: unlock it, thread it through the barrel, bring it back around, put it in and lock it back,” Mr Holmes told WLS-TV, explaining how the locks work.

“If you leave it, secure it. This could have been prevented.”