Samuel Kunz was third on the Simon Wiesenthal Center's list of most wanted Nazi war crimes suspects and had been due to go on trial early next year.
He was also accused of personally shooting dead 10 Jews at the camp in occupied Poland during 1942-43.
State Prosecutor Andreas Brendel said Kunz died at home last Thursday.
The cause of death was not clear.
SS training camp
Kunz had admitted working at Belzec and had been called as a witness in the trial of alleged Nazi death camp guard John Demjanjuk, 90, who was deported from the US in 2009.
Prosecutors allege that both men trained at the SS camp at Trawniki.
Kunz was accused of moving Jewish victims from trains at Belzec, pushing them into gas chambers and throwing their bodies into mass graves.
He was also alleged to have shot dead two people who had escaped from a train and killed eight others who had been wounded.
Mr Brendel, who is head of the Dortmund-based centre for investigating Nazi war crimes, told the BBC News website that Kunz had been due to go on trial in January or February next year.
He said that his department had spent the whole year working on the case. He added that work was continuing on a number of other cases but it was not clear whether any would come to court.
Efraim Zuroff, head of the Simon Wiesenthal Center's unit for hunting down Nazi war criminals, said it was very important that Kunz had been indicted.
"At least a small measure of justice was achieved," he said.
Mr Demjanjuk, 90, went on trial last year on charges of assisting in the murder of 28,000 Jews at the Sobibor death camp.
He denies the charges.
BBC News
