Independent Cambodian broadcaster Mam Sonando was handed the lengthy jail term in Phnom Penh Municipal Court.
The U.S. State Department said in a release that it is "deeply concerned by the conviction and harsh sentence."
"A number of observers in Cambodia have noted that the charges against him appear to have been politically motivated, based on his frequent criticism of the government," the State Department said.
"We strongly support freedom of expression, including the rights of members of the press to peacefully express political views without fear of persecution, retribution or intimidation, consistent with the International Covenant on Civil and Political Rights and the Universal Declaration of Human Rights.
"We call on the Cambodian government to release Mam Sonando immediately, to ensure that its court system is free from political influence, and to reaffirm its commitment to guaranteeing its citizens' basic human rights."
Reporters Without Borders, which has its headquarters in France, and the Cambodian Center for Independent Media issued a statement expressing their displeasure with the punishment given the 71-year-old Radio Beehive owner for allegedly inciting unrest in the southeastern province of Kratie in mid-May.
"Freedom of expression and media freedom are in danger in Cambodia," the two organizations said. "We call on the authorities to release Mam Sonando at once and to quash this conviction on charges for which there is no evidence."
Sonando was arrested in July at the behest of Cambodian Prime Minister Hun Sen and was found guilty of "insurrection" and "inciting the use of arms against the state" regarding a supposed uprising in Kratie aimed at creating "a state within the state."
Sonando was out of the country during the unrest.
He previously was arrested in 2003 and 2005 for allegedly defaming the prime minister.
The journalists' organizations expressed concern for the safety of journalists and rights activists in Cambodia, noting journalist Hang Serei Oudom was slain in mid-September after implicating local officials in illegal trafficking in timber, and journalist Ek Sokunthy and his wife told the Phnom Penh Post last Wednesday they had been attacked by a former police officer and two other men for no apparent reason.
Reporters Without Borders ranks Cambodia 117th out of 179 countries in its 2011-2012 press freedom index.