Alex Gonzalez-Davidson, who was a tireless anti-dam campaigner in Cambodia, was deported on a Monday night flight, said Gen. Khieu Sopheak, an Interior Ministry spokesman.
"His visa had expired and he refused to leave Cambodia as ordered from the ministry, therefore we had no choice but to deport him," he said.
Gonzalez-Davidson was a co-founder of non-governmental group called Mother Nature, which has vocally opposed the construction of a controversial hydropower dam in southwestern Cambodia's Koh Kong province. Cambodia has signed a deal to build the mega-dam with China's state-owned Sinohydro.
The group says the dam would force hundreds of families from their homes and destroy the natural habitat across a vast expanse of one of Southeast Asia's last great wilderness areas in the Cardamom Mountains.
Gonzalez-Davidson's visa expired Friday and the government refused to extend or renew it. A week earlier, the Ministry of Interior had sent a letter to the Spaniard to inform him that he must leave Cambodia after the expiration.
Before the deportation Monday, Prime Minister Hun Sen publicly appealed to Gonzalez-Davidson to leave the country, prompting the Spaniard to say he would remain with or without a visa.