Philippe Lauwers was found guilty of "committing an indecent act against an underaged boy in late 2011", Judge Phong Vann at southern Kandal provincial court told AFP, adding that the boy was now aged 16.
The Frenchman will walk free in two months having already served a year in jail, unless his victim appeals the ruling, the judge added.
Lauwers has been held in custody since his arrest in January 2012 on suspicion of sexually abusing three boys, although two of them later dropped their complaints.
He was ordered to pay the boy $1,000 in compensation and will be deported after his release.
Samleang Seila, country director for child rights group Action Pour Les Enfants, said the sentence was "too lenient".
"We cannot accept it. The lawyer will consult with the boy whether to appeal the verdict," he said, adding Lauwers should instead have been charged with buying sex, which carries a minimum seven year tariff.
Cambodia launched an anti-paedophilia drive in 2003 in an effort to end its reputation as a haven for child sex offenders.
Dozens of foreigners have since been jailed for child sex crimes or deported to face trial in their home countries.