Authorities said the toddler was taken to Le Bonheur Children's Hospital in critical condition. That child later died from his injuries.
Investigators confirmed the shooting happened inside one of the apartments, and there were adults inside when it happened.
Police said two toddlers – including the boy who was killed – were playing alone in a room at the time.
Investigators did not specify how the gun ended up going off, and it is unclear whether the shooting was accidental.
A 4-year-old boy died Monday after he was shot at an apartment complex in Memphis, Tennessee, according to police.