Prosecutors believe the Ukrainian-born Demjanjuk helped herd tens of thousands of Jews and others into the gas chambers while a guard at the Sobibor death camp in Nazi-occupied Poland in 1943.
Demjanjuk, the Nazi-hunting Simon Wiesenthal Centre's number one suspect among those known to be alive, was deported from the United States in May after losing a lengthy legal battle.
No date was set for the trial to begin, and the defendant's poor health means that he will be subjected to no more than two court sessions of 90 minutes each day.
The octogenarian, who moved to the United States and worked as an auto mechanic in Ohio after World War II, suffers from kidney disease, arthritis and cancer, according to his family. His son John Demjanjuk Jr. has said that German doctors had given his father about 16 months to live because of bone marrow disease.
Courts in Israel and the United States have previously stated he was a guard at Sobibor, accusations he had never previously challenged, but his lawyer now says he was never there.
Prosecutors also have an SS identity card with a photograph of a young man said to be Demjanjuk and written transcripts of witness testimony placing him at the camp.
Demjanjuk spent five years on death row in Israel before being acquitted in 1993 when the Jewish state's highest court overturned the verdict. In that case, Demjanjuk was suspected of being "Ivan the Terrible," a particularly brutal camp guard who specialised in hacking at naked prisoners with a sword, but Israel established it had the wrong man.
Demjanjuk is stateless, having been stripped of his US citizenship for lying about his past. Munich prosecutors say it falls on the German city to try him because he had been registered as living there after World War II.
During months of legal wrangling that preceded his eventual deportation from his home in Cleveland, Ohio, the US Justice Department rejected the family's argument that he would not survive the flight to Germany. It released four secretly filmed surveillance videos showing him apparently getting out of a car without difficulty.
This contrasted sharply with the scene before his deportation when he was carried by federal agents in a wheelchair, moaning and sobbing. Last week, Germany's top court rejected an attempt by Demjanjuk's defence team to declare his deportation illegal.
Ephraim Zuroff, director of the Wiesenthal Centre office in Jerusalem, said he was "very happy that the court case is moving forward."
"The case is communicating a very important message that those guilty of war crimes can still be prosecuted even now and that they are just as guilty today as they were decades ago," Zuroff said.
Demjanjuk's trial is set to be one of the last in Germany to deal with the crimes of more than six decades ago.
On July 7, a German court declared an 88-year-old former Nazi SS soldier of Dutch origin well enough to stand trial for the murder of three Dutch resistance fighters, overturning an earlier decision.
Former army commander Josef Scheungraber, 90, meanwhile, is currently on trial in Munich, charged with murdering 14 civilians in the Tuscan village of Falzano on June 26, 1944.