John Demjanjuk (born Ivan Mykolaiovych Demjanjuk; Ukrainian: Іван Миколайович Дем'янюк; 3 April 1920 – 17 March 2012) was a Ukrainian-American who served as a Trawniki man and Nazi camp guard at Sobibor extermination camp, Majdanek, and Flossenbürg. Demjanjuk became the center of global media attention in the 1980s, when he was tried and convicted in Israel after being misidentified as "Ivan the Terrible", a notoriously cruel watchman at Treblinka extermination camp. In 1993 the verdict was overturned. Shortly before his death, he was tried and convicted in Germany as an accessory to 28,000 murders at Sobibor.

Born in Soviet Ukraine, Demjanjuk was conscripted into the Red Army in 1940. He fought in World War II and was taken prisoner by the Germans in spring 1942. He was recruited by the Germans and trained at Trawniki concentration camp, going on to serve at Sobibor extermination camp and at least two concentration camps. After the war he married a woman he met in a West German displaced persons camp, and emigrated with her and their daughter to the United States. They settled in Seven Hills, Ohio, where he worked in an auto factory and raised three children. Demjanjuk became a US citizen in 1958.

In August 1977, Demjanjuk was accused of having been a Trawniki man. Based on eyewitness testimony by Holocaust survivors in Israel, he was identified as the notorious Treblinka extermination camp guard known as "Ivan the Terrible". Demjanjuk was extradited to Israel in 1986 for trial. In 1988, Demjanjuk was convicted and sentenced to death. He maintained his innocence, claiming that it was a case of mistaken identity. In 1993 the verdict was overturned by the Israeli Supreme Court, based on new evidence that cast reasonable doubt over his identity as "Ivan the Terrible". Although the judges agreed that there was sufficient evidence to show that Demjanjuk had served at Sobibor, Israel declined to prosecute. In September 1993 Demjanjuk was allowed to return to Ohio. In 1999, US prosecutors again sought to deport Demjanjuk for having been a concentration camp guard, and his citizenship was revoked in 2002. In 2009, Germany requested his extradition for over 27,900 counts of acting as an accessory to murder: one for each person killed at Sobibor during the time when he was alleged to have served there as a guard. He was deported from the US to Germany in that same year. On 12 May 2011, he was convicted and sentenced to five years in prison.

According to legal scholar Lawrence Douglas, in spite of serious missteps along the way, the German verdict brought the case "to a worthy and just conclusion". After the conviction, Demjanjuk was released pending appeal. He lived at a German nursing home in Bad Feilnbach, where he died on 17 March 2012. Having died before a final judgment on his appeal could be issued, under German law, Demjanjuk remains technically innocent. In January 2020, a photograph album by Sobibor guard Johann Niemann was made public; some historians have suggested that a guard who appears in two photos may be Demjanjuk.