Groening was convicted in Lueneburg in 2015 as an accessory to the murder of 300,000 Jews.
Prison authorities have yet to provide Groening's death certificate.
He was found guilty in July 2015 of being an accessory to the murders of 300,000 people at the camp, and sentenced to four years in prison.
The 96-year-old former "Bookkeeper of Auschwitz" died before starting his jail sentence for participation in the Holocast, according to a Sunday report.
He had been expected to enter prison at the end of January, but the continued wrangling over the state of his health kept him from beginning to serve his sentence.
He was not involved in the actual killings, but counted cash taken from victims on their arrival at the camp.
His trial went to the heart of the question of whether people who were minor participants in the Nazi atrocities, but did not actively participate in the killing of 6 million Jews during the Holocaust, were themselves guilty.
Oskar Groening as a young man in an SS uniform.
In December 2017, the Federal Constitutional Court of Germany decided that Groening was liable and his punishment must begin.
At least 1 million people were killed at Auschwitz concerntration camp in Nazi-occupied Poland.
His case followed the conviction of former concentration camp guard John Demjanjuk, who was sentenced because he had served at the Sobibor camp in occupied Poland - rather than for crimes he was known to have committed.
Prisoners were transported by train to the Auschwitz death camp.
German prosecutors say they have been notified that a former guard dubbed the "accountant of Auschwitz" has died.
He died in a hospital on Friday, according to Spiegel Online.