Twenty parties were running against Hun Sen's ruling Cambodian People's Party (CPP) but the main opposition has been silenced. There were calls for a boycott on the grounds the vote is neither free nor fair.
Polls closed Cambodia on Sunday with the 65-year-old Hun Sen expected to stay in power for at least two more five-year terms.
The only credible opposition, the Cambodian National Rescue Party, was dissolved last year by the Supreme Court. The party called on its supporters to boycott the poll.
Hun Sen and his wife cast their ballots south of the capital shortly after polling stations opened. Some 8.3 million people are registered to vote.
Russia, China and Indonesia sent election observers but the US, EU and Japan did not, saying that the elections were not credible.
Ahead of the vote, media and websites were controlled or blocked as the government claimed the information disseminated might affect security.
Cambodia Democracy Act
The US Congress passed the Cambodia Democracy Act last week which aims to promote democracy and impose sanctions on Hun Sen's "inner circle."
Hun Sen has been in power for 33 years, which makes him the world's longest ruling national leader.
At the last elections in 2013, the opposition claimed manipulation of the voter registration process had prevented a victory for them.
Hun Sen was a member of the communist Khmer Rouge which toppled a pro-US government in the 1970s before leaving for Vietnam during the regime of Pol Pot which left nearly two million people dead.
In 1985, Hun Sen was backed by Vietnam and led a six-year civil war against the Khmer Rouge until the 1991 Paris Peace Accords which brought a democratic political framework.
Polls closed at 3 p.m. local time (0800 UTC) and first results are expected on Sunday night.
jm/bw (Reuters, AFP)