South Salt Lake Fire Chief Steve Foote says the boy died of smoke inhalation Monday.
Authorities say the fire at the Royal Gardens Apartments was reported a short time after noon and that fire crews responded quickly, extinguishing the fire a short time later.
The boy's mother and brother were being treated for smoke inhalation. The 10-year-old's name was not immediately released.
The Salt Lake Tribune reported on its Web site Monday that at least two apartments at the two-story complex were destroyed and more than a dozen people immediately evacuated the complex.
Personnel from several agencies responded to the fire.