Mirra, 41, was found sitting in a truck with what appeared to be a self-inflicted gunshot wound, the Greenville Police Department said. The department is investigating the incident.
Mirra held the record for most X Games gold medals for several years, and was one of the most prominent athletes in the sport. He is survived by his wife and two children.
"We mourn the loss today of a great friend and wonderful human being who touched the lives of so many around the world with his gift," Greenville Mayor Allen Thomas said in the police statement.