Hugo Leuthold, who had already been jailed once in Cambodia on similar charges, was arrested in the capital Phnom Penh in March.
"The Phnom Penh Municipal Court sentenced him to five years' imprisonment and ordered that he be expelled from the country after serving the sentence," Leuthold's lawyer Dun Vibol told AFP, adding his client denied the charges and will appeal the sentence.
Rights group Action Pour Les Enfants (APLE) said it was Leuthold's second conviction for child sex crimes in Cambodia after he was given two years for sexually abusing four boys, aged between 11 and 13, in 2010.
He served only 10 months in jail, APLE said, adding there were no details as to why he was not deported after his release.
The court ordered him on Friday to pay $500 to the victim in compensation, according to APLE, which provided legal counsel for the boy.
Leuthold was also arrested in September 2010 in Thailand on suspicion of molesting a 12-year-old boy at a Thai seaside resort but was later granted bail, APLE said.
Dozens of foreigners have been jailed for child sex crimes or deported to face trial in their home countries since Cambodia launched an anti-paedophilia push in 2003 in a bid to shake off its reputation as a haven for sex predators.