A statement issued by the National Assembly said that all 115 members voted in favour of the law, which aims to prevent the recurrence of such events and to provide justice for victims.
Under the new law, people denying the genocide can face up to five years in prison and a fine of between $US2500 ($A3,932) and $US125,000 ($A196,587).
In 2013, Cambodia enacted a similar law against denying Khmer Rouge atrocities after then-Prime Minister Hun Sen called for the measure.
He claimed that a leading opposition lawmaker suggested that some of the evidence of Khmer Rouge atrocities was fabricated by Vietnam.
That law prescribed prison terms of six months to two years and fines of up to $US1,000 ($A1,573).
Critics charged that the measure was a veiled attempt to undermine his political opponents and allege that the latest proposed legal change has a similar motive.
The latest National Assembly statement said the draft law is designed to punish individuals who do not recognise, or deny or object to the recognition of crimes that were proven in legal proceedings held by a UN-backed tribunal to have been committed under the Khmer Rouge.
The tribunal, which held a series of trials starting in 2009, found that the Khmer Rouge government committed genocide, crimes against humanity and grave breaches of the 1949 Geneva Conventions.
The new law's adoption comes two months ahead of the 50th anniversary of the Khmer Rouge takeover of Cambodia on April 15, 1975 after five years of civil war.
The bill will now be sent to the Senate for approval, a pro forma action, and will become effective when it is signed by King Norodom Sihamoni.
The Khmer Rouge, under the leadership of the late Pol Pot, stayed in power until 1979 when it was ousted by an invasion from neighbouring Vietnam.
Its radical policies while in power are thought to have been responsible for an estimated 1.7 million deaths from starvation, execution, and illness.
Aside from the few surviving senior leaders of the Khmer Rouge and other aging veterans of their movement, few Cambodians who lived through those times would deny that the Khmer Rouge were responsible for severe human rights violations on a massive scale.
In May 2024, Hun Sen — who himself had been a middle-ranking commander with the Khmer Rouge before defecting — suggested that the 2013 law needed to be updated.
His government was noted for intimidating its opponents through the courts, which are widely seen by rights groups as allies of the ruling party.
His eldest son Hun Manet took over as prime minister in 2023 after Hun Sen wrapped up a 38-year reign as Cambodia's autocratic leader.
