The 101-year-old, named only as Josef S under German law, was convicted of 3,518 counts of accessory to murder for serving at the Sachsenhausen concentration camp.
The Neuruppin Regional Court sentenced him to five years in prison.
The man, who was not identified, had denied working as an SS guard at the camp and aiding and abetting the murder of thousands of prisoners.
He was complicit in the shooting of Soviet prisoners of war and the murder of others with Zyklon B gas.
The defence had called for his acquittal and is set to appeal against the prison sentence.
Tens of thousands of people died at Sachsenhausen during World War Two from starvation, forced labour, medical experiments and murder by the SS. More than 200,000 people were imprisoned there, including political prisoners as well as Jews, Roma and Sinti Gypsies.
Josef S said that he had worked as a farm labourer near Pasewalk in north-eastern Germany during the period in question but his true role was revealed in official documents.
The court ruled that he worked at the camp on the outskirts of Berlin between 1942 and 1945 as an enlisted member of the Nazi Party’s paramilitary wing.
“The court has come to the conclusion that, contrary to what you claim, you worked in the concentration camp as a guard for about three years,” presiding Judge Udo Lechtermann said, according to dpa, adding that in doing so, the defendant had assisted in the terror and murder machinery of the Nazis.
“You willingly supported this mass extermination with your activity,” Judge Lechtermann said.
Prosecutors had based their case on documents relating to an SS guard with the man’s name, date and place of birth, as well as other documents.
For organisational reasons, the trial was held in a gymnasium in Brandenburg/Havel, the 101-year-old’s place of residence.
The man was only fit to stand trial to a limited extent and was only able to participate in the trial for about two and a half hours each day. The trial was interrupted several times for health reasons and hospital stays.