Braylon Robinson was playing at a house in Cleveland, Ohio, under the watchful eye of his mother.
According to reports, she was also babysitting her friend’s two children.
She was in the same room as the kids - and had apparently only just turned her back - when the three-year-old boy found an unattended gun.
He then pulled the trigger, sending a bullet into the one-year-old baby's face.
Minutes later, the baby was rushed to a hospital in the area, where he was pronounced dead.
"The mother observed the victim lying on the floor bleeding. A handgun was recovered from the room near the 3 year old child," police said in a statement.
Now Trashawn Robinson, a cousin to Braylon, has spoken out about the tragedy to NBC News, teling them the baby was “always smiling” during his short life.
"He kept everyone happy… [but] he didn’t even make it to the hospital.
“We're torn apart. Everybody loved Braylon."
The cousin said he did not know who owned the gun. Police have said the adult responsible for the weapon could now be charged.
Speaking outside the house after the shooting, Cleveland Police Chief Calvin Williams told reporters that more needs to be done to fight gun crime.
"It's a sad day for Cleveland," he said.
"This fascination that we have with handguns, not just in this city but in this country, has to stop. This is a senseless loss of life."
Little Braylon is, tragically, the 24th child to have been killed as a result of accidental gun violence in 2015 alone.
Our thoughts are with his family at this sad time.