A Californian judge has ordered the Jehovah's Witnesses to pay $13.5 million to a man who was sexually abused in the 1980s by a church leader, the attorney for the victim said.

Jehovah's Witnesses elders had assigned a man to work with Jose Lopez on bible studies, even though they knew he had admitted to molesting another boy in 1982, because they felt he was "repentant", Lopez's attorney Irwin Zalkin said.

Lopez, who filed the civil suit in 2010, first went to the Spanish-speaking Kingdom Hall church in San Diego in 1985 at the age of seven, according to the complaint.

Mario Moreno, a lawyer for Jehovah's Witnesses - whose parent group is the Watchtower Bible and Tract Society of New York - told the San Diego Union-Tribune newspaper that the church will appeal Superior Court Judge Joan Lewis' judgment.

"The trial judge's decision is a drastic action for any judge to take given the circumstances of this case," the statement in the paper said. "Jehovah's Witnesses abhor child abuse and strive to protect children from such acts."

The US-based Jehovah's Witnesses number about 8 million worldwide and are known for their foreign ministries as well as their door-to-door campaigns.