Police said in a statement Saturday that officers and paramedics found the toddler with a bullet wound to the torso after they were called to the boys' home in the San Fernando Valley.
Police say the 2-year-old died at a hospital Friday night.
The statement says homicide detectives interviewed family members and determined that the gun had gone off while the 9-year-old was playing with it.
Police say no criminal charges are pending. The name of the 9-year-old was not released because he is a minor, and the name of the 2-year-old is being withheld while family members are properly notified.
