"We will appeal against the [court's] decision," Kar Savuth, Duch's lawyer, told AFP by telephone.
He had said previously that Duch, a former Math teacher, wanted to be acquitted on the grounds that he was not a senior member of the Khmer Rouge hierarchy.
Prosecutors, who had sought a 40-year jail term from the tribunal, have said they are considering whether to appeal against the reduced jail time given by the court.
Passing sentence on Monday the judges in the special court reduced Duch's jail sentence on the grounds that he had been detained illegally for years before the tribunal was established.
Judge Nil Nonn shaved 16 years off the 35-year sentence for time already served and for illegal detention in a military prison.
But Andrew Cayley, an international co-prosecutor, told AFP on Tuesday he thought it was a "logical, well-reasoned judgment".
"You must recall that this is a man who actually acknowledged responsibility and pleaded guilty but still received a sentence of 35 years, which is actually on the high side as far as guilty pleas are concerned," Cayley said.
Led by Pol Pot, the so-called Khmer Rouge "Brother Number One", the movement wiped out nearly a quarter of Cambodia's population through starvation, overwork and execution in the notorious "killing fields".
Pol Pot died in 1998 but four other Khmer Rouge leaders, all said to be more senior than Duch, are in custody awaiting trial.
"Brother Number Two" Nuon Chea, former head of state Khieu Samphan, the former foreign minister Ieng Sary and his wife Ieng Thirith, who was the minister of social affairs, are expected to go on trial next year.