Nuon Chea, 92, known as Brother Number Two, was found guilty of genocide relating to crimes committed against the Cham Muslims and ethnic Vietnamese by the UN-backed Extraordinary Chambers in the Courts of Cambodia, reports Efe news.
Khieu Samphan, 87, was found guilty of genocide "by joint criminal enterprise" in relation to the killings of ethnic Vietnamese, the chamber said.
The pair were also found of crimes against humanity.
The landmark ruling is the first time that a court has found that genocide was committed by the regime.
An estimated 2 million people - around a quarter of the population - died from execution, starvation and forced labour during the regime's rule from 1975-1979 in which it set out to establish an agrarian utopia.
Nuon Chea and Khieu Samphan had already been convicted and sentenced to life in prison in 2014 for crimes against humanity related to forced evacuations in the first of their case's two trials.
The second trial, which began in 2014 and concluded last June, focused on allegations of genocide of Cham Muslims and ethnic Vietnamese, grave breaches of the Geneva Conventions, forced marriage and rape, purges, persecution of Buddhists, and crimes against humanity at security centres, worksites and a cooperative.
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