Jeffrey Walker, 29, was arrested late Tuesday, shortly after Jadarrius Speights died, the Tampa Bay Times reported.
Debbie Carter, a spokeswoman for the Hillsborough County Sheriff's Office, said Walker, who had a concealed carry permit, bought the handgun at a gun shop. He allegedly left it in a backpack in the room he shared with Jadarrius.
Walker was charged with culpable negligence. The boy's parents, Jasmine Bell, 21, and Trentin Speights, 22, were in the apartment in another room when Jadarrius found the gun, Carter said.
Toddler uninjured after being knocked over by Obama family dog
Texas principal bans speaking Spanish, stirs controversy