Kaing Guek Eav, alias Duch, was convicted of war crimes and crimes against humanity in July and sentenced to 35 years in prison — reduced to 19 years by time served and other factors.
Prosecutors already have appealed for a longer sentence, saying the judgment "gives insufficient weight to the gravity of Duch's crimes."
Duch is the only senior Khmer Rouge member held by the tribunal to admit guilt and express remorse. But in a move that stunned judges and onlookers, he and his lawyer on the last day of his trial asked that he be acquitted and freed.
The ruling by a U.N.-backed tribunal marked the first trial of a major Khmer Rouge figure more than three decades after the regime's brutal rule in the 1970s led to the deaths of an estimated 1.7 million people from execution, disease, starvation and overwork.
Duch's lawyer Kar Savuth said the appeal argues Duch should be acquitted due to legal errors made by the tribunal and because it lacked jurisdiction. Duch's lawyers say he was one of 196 prison chiefs and merely following orders.
Both Duch's appeal and the prosecutor's filing would be decided by the tribunal's Supreme Court chamber.
Duch (pronounced DOIK) supervised the notorious S-21 prison in Phnom Penh where as many as 16,000 people were tortured before being executed.
Survivors have expressed anger that the sentence was too weak and could allow a former key player in the genocide to one day walk free.
The judges had said that they took into consideration the historical context of the atrocities: The 1975-79 regime was the product of the Cold War times.
They also recognized that Duch — unlike any of the others in detention — was not in the Khmer Rouge's inner circle, had cooperated with the court and shown expressions of remorse, however "limited."
But they flatly rejected claims he was acting on orders from the top or that he was a "cog in the machine" who could not get out.
Four more defendants are expected to go on trial early next year: Nuon Chea, the Khmer Rouge ideologist; former head of state Khieu Samphan; then-foreign minister Ieng Sary; and his wife Ieng Thirith, who was minister for social affairs.