With 18 parties registered to contest the election, including incumbent Prime Minister Hun Sen's ruling Cambodian People's Party (CPP), the vote is expected to be a repeat of the 2018 elections, in which the CPP won all seats in the National Assembly.
Cambodia's last remaining opposition party, the Candlelight Party, was barred from the election by the Constitutional Council in May on a registration technicality.
Sen, who is also running for prime minister, has held the position in Cambodia and its predecessor states since 1985 and is likely to remain the country's ruler for another five years.
Critics have called the vote a fraud, as Sen, who has been in power for 38 years, does not appear to have any legitimate political challengers.
"It's a rigged election because there are no real strong opposition parties," one voter, an aid worker in Phnom Penh, told a British news outlet earlier this week.
Sen has also showed signs of handing power to his son, Hun Manet, possibly within weeks, as Sen has become increasingly authoritative, political analysts say.