Hannover prosecutor Kathrin Soefker said a lawyer informed her office that Oskar Groening, 96, died Friday in a hospital.
He was convicted in Lueneburg in 2015 as an accessory to the murder of 300,000 Jews.
In December 2017, the Federal Constitutional Court of Germany decided that Groening was liable and his punishment must begin.
Zuroff pointed out that two other Nazi war criminals - John Demjanjuk, who was convicted in 2011, and Reinhold Hanning, who was convicted in 2016 - also died while awaiting decisions on their appeals. Groening then filed an appeal to skirt his prison sentence due to his age and poor health. This was one of the last Holocaust-related court trials.
Mr Groening's court case was seen as one of the last major cases related to the Holocaust, where some 6 million Jews were murdered by Adolf Hitler's regime.
Groening previously described some of the murders that he witnessed at Auschwitz. An officer shut the door, put on a gas mask, opened a can and poured its contents down a hatch.
Around 1.1 million people, mostly Jews from across Europe, were killed at the Auschwitz-Birkenau complex between 1940 and 1945.
"The screams became louder and more desperate but after a short time they became quieter again", Groening said. He was first noticed when doing interviews in 2005, admitting his wrongdoings in an attempt to persuade Holocaust deniers to accept the truth, according to Reuters.
A clemency plea filed by Groening's attorney with German prosecutors was rejected in January, and earlier this month he asked the justice minister of Lower Saxony, the northern German state where his 2015 trial took place, to pardon him so that he did not have to go to prison.
At his trial he said he was "morally guilty" for the work he carried out.