Comrade Duch, the former head of the Khmer Rouge's notorious torture and execution prison known as S-21, was Monday sentenced to 35 years after being found guilty of crimes against humanity and war crimes.
But the court ruled that it would deduct 16 years from that sentence, meaning the condemned war criminal will serve an effective 19 years.
Duch, whose real name is Kaing Guek Eav, has 90 days to appeal.
In his verdict to a packed courtroom, judge Nil Nonn said Duch would benefit from an 11-year deduction for time already served while awaiting trial.
And he would get a further five-year deduction as compensation for being held illegally in a Cambodian military prison between his arrest in 1999 and his transfer to the UN-backed tribunal in 2007.
'The chamber has noted a number of aggravating features, including the shocking and heinous character of the offences, which were perpetrated against at least 12,273 victims over a prolonged period,' judge Nil Nonn told the court.
'Such factors, when considered cumulatively, warrant a substantial term of imprisonment,' he said, before adding that Duch's cooperation, acceptance of responsibility, and 'limited expressions of remorse' would serve to mitigate.
The prosecution said later it would examine the judgment before deciding whether to appeal.
The verdict was criticised by many victims, including Theary Seng, who lost family members under the Khmer Rouge.
'That is not acceptable. What is unacceptable is to envision him as a free man even for one minute in the public sphere,' she said.
Duch's 77-day trial heard shocking testimony from some of the prison's few survivors and from family members of people murdered under Duch's command. Former guards and torturers also gave evidence.
An estimated 20,000 people were tortured and condemned to death at S-21 as perceived enemies of the regime during the movement's rule of Cambodia between 1975-79.
In Monday's judgment the court rejected Duch's argument that he had acted under duress, and said his claim to have been merely following orders was not a legitimate defence.
The court also said he bore individual criminal responsibility for a number of offences related to crimes against humanity, including murder, extermination, enslavement, imprisonment, torture, persecution on political grounds, and other inhumane acts.
He was also guilty of grave breaches of the Geneva Conventions, including wilful killing, torture, wilfully causing great suffering or serious injury, wilfully depriving a prisoner of war or civilian of the right to a fair trial, and unlawful confinement.
The tribunal's remit is to try senior surviving Khmer Rouge leaders and those most responsible for crimes committed by the regime. Four former leaders are in detention for their alleged involvement in the deaths of 1.7 million people from execution, disease, starvation and overwork.
The four, whose trials are expected to start early next year, are: Brother Number Two Nuon Chea, the movement's ideologue; head of state Khieu Samphan; foreign minister Ieng Sary; and his wife, the social affairs minister Ieng Thirith.
The movement's leader, Pol Pot, died in 1998.