Alexander Trofimov, a Russian national also known as Stanislav Molodyakov, was arrested Monday in Kandal province's Ponhea Leu commune, The Phnom Penh Post reported.
"We have been trying to arrest him for months. As of now, he's in detention at the Department of Immigration. We must deport him soon following the order from the [Interior] minister. But we are now discussing about the deportation," National Police spokesman Kirth Chantharith said.
Activists said they welcome the arrest and deportation, but say it didn't come soon enough.
Trofimov was convicted of sexually abusing 17 Cambodian girls from 2005 to 2007. In December, he was given a royal pardon and released from prison after serving just half of his eight-year sentence. He is also wanted by Interpol for allegedly raping six girls in Russia.
"This sends a message that there should be an amendment to the law … to make it mandatory for any child sex convict to be deported from the country," said Samleang Seila, director of Action Pour Les Enfants.
Phil Robertson, deputy director of Human Rights Watch's Asia division, said Trofimov should be extradited to Russia for prosecution there, but said further questions remained about how Trofimov eluded the authorities for so long.
"The Cambodian government cannot just wash its hands by deporting Trofimov and calling the case closed. There also needs to be a serious and impartial investigation about how these events came to pass," he said.