Kaing Guek Eav, alias Duch, was convicted of war crimes and crimes against humanity but was sentenced last month to serve just 19 years in prison.
A statement by the prosecutors said the judgment "gives insufficient weight to the gravity of Duch's crimes and his role and his willing participation in those crimes."
Separately, the prosecutors said they were seeking to lodge conspiracy charges against four senior former Khmer Rouge leaders who are in custody but have yet to be tried.
The tribunal is seeking justice for the estimated 1.7 million people who died from execution, overwork, disease and malnutrition as a result of the radical policies of the 1975-79 communist Khmer Rouge regime.
Duch (pronounced DOIK) was the first defendant to be tried. He supervised a prison where as many as 16,000 people were tortured before being executed.
Also found guilty of torture and murder, Duch was originally sentenced to 35 years. Time served reduced the sentence 11 years, and he was compensated five years for illegal detention in a military prison. The length of the sentence was widely criticized.
Prosecutors argued that mitigating circumstances were given undue weight and added their view was that "Duch should be separately convicted of the crimes against humanity of enslavement, imprisonment, torture, rape, extermination, and other inhumane acts."
The appeal was filed Monday, and the prosecutors' statement said substantive arguments supporting it would be filed within the required 60 days.
Four more defendants are expected to go on trial early next year: Nuon Chea, the Khmer Rouge ideologist; Khieu Samphan, its former head of state; Ieng Sary, its foreign minister; and his wife Ieng Thirith, who was minister for social affairs.
In a separate announcement, the prosecutors announced that they had made their final submission to the court on Monday of the case against the four, which in addition to the charges for which Duch was tried, includes "joint criminal enterprise."
The doctrine of joint criminal enterprise makes defendants responsible for the actions of co-defendants on the basis of conspiracy. Such a charge could allow a conviction in cases where a defendant might not be directly linked to commission of a crime under the other charges.
The announcement said the accused conspired "to enforce a political revolution in Cambodia and systematically destroy any opposition," and that through the conspiracy they "enslaved the Cambodian population, deprived them of their fundamental human rights and freedoms and orchestrated mass killings of individuals" whom they regarded as enemies.
The tribunal's co-investigating judges will evaluate which charges to proceed with. The prosecutors said their submission includes a 931-page summary of the facts supporting their allegations, and that the charges are supported by more than 2,800 documents of evidence, including statements from witnesses.