Chan Madina, a judge at the Phnom Penh Municipal Court, told The Associated Press she sentenced 46-year-old Myron Maboris on Monday. She convicted him of debauchery during a trial last Thursday.
Debauchery is a broadly defined Cambodian legal term for sexual offenses against minors that is punishable by 10 to 20 years in jail.
She said she also ordered the man to pay 3 million riel (US$740; EUR550) in compensation to the girl.
A U.S. Embassy spokesman declined to comment on the case, citing privacy laws.
Police arrested Maboris during a raid on his Phnom Penh guest house room in October last year while he was with the ethnic Vietnamese girl, who was naked.
Police also arrested the girl's 49-year-old mother and three men - a motorbike taxi-driver who transported her and two alleged sex brokers.
The girl's mother was also sentenced to 13 years in jail for involvement in the crime, Chan Madina said, adding the motorbike taxi-driver and two other men each received eight-year jail terms on conspiracy charges.
Lax law enforcement and poverty have made Cambodia a prime destination for foreigners seeking sex with minors. But police have recently stepped up efforts to fight the crime.