The lawsuit was filed on Friday in Los Angeles federal court on behalf of Meach Sovannara and his family.
Meach, who lived in Long Beach, California, was spokesman for the opposition Cambodia National Rescue Party.
He was arrested in 2014 during a demonstration and is serving 20 years in Cambodia for insurrection. His wife and three children remain in Long Beach.
The lawsuit alleges torture through illegal detention and acts of international terrorism by the Cambodian government and the prime minister's son, Hun Manet, who heads the military and security police.