Prosecutors had said that Josef S, a member of the Nazi party's paramilitary SS, helped to send 3,518 people to their deaths at the Sachsenhausen camp, north of Berlin, by regularly standing guard in the watchtower between 1942 and 1945.
The trial took nearly nine months as doctors had said the man, whose full name was not disclosed due to German trial reporting rules, was only partially fit to stand trial, and sessions were limited to two and a half hours a day.
Some people interned in Sachsenhausen were murdered with Zyklon-B, the poison gas also used in other extermination camps where millions of Jews were killed during the Holocaust.
Sachsenhausen housed mainly political prisoners from across Europe, along with Soviet prisoners of war and some Jews.
There has been a spate of charges brought against former concentration camp guards in recent years for World War Two crimes against humanity. In September, a former camp secretary went on the run the day her trial was to begin, but was caught by police hours later.
A 2011 court ruling paved the way for these prosecutions, stating that even those who contributed indirectly to wartime murders, without pulling a trigger or giving an order, could bear criminal responsibility.
Defence lawyer Stefan Waterkamp said his client would appeal Tuesday's ruling and that a higher court would decide whether "general guard service without concrete participation" provided sufficient grounds for such a verdict.