Nazi (Nazi Party) German extermination camp on the Ner River, a tributary of the Warta, in western Poland. It opened in December 1941 and closed in January 1945 and was operated to execute Jews, most of whom were Polish. Some Soviet prisoners of war and Roma (Rom) (Gypsies) were also executed here. Estimates of the number executed range from 170,000 to 360,000.

As the first camp to conduct mass executions of Jews, Chelmno introduced the most deadly phase of the Holocaust. Its victims were from the Łódź ghetto and from 36 communities in western Poland. Also sent to Chelmno were 88 children from the Czech town of Lidice, whose population was decimated as a collective reprisal for the assassination of Reinhard Heydrich (Heydrich, Reinhard) in May 1942.

The facilities included three gas vans and two crematoria that were 32.5 feet (10 metres) wide and 16–19 feet (5–6 metres) long. Gassing began on December 8, 1941, just hours after Pearl Harbor was struck by Japanese planes. For the Nazis, the drawback of the vans was their limited capacity. They could not handle large numbers of victims. They were also slow. The excruciating suffering of the victims, who died by asphyxiation, was of little concern to the SS, but the task of unloading the vans after each use was time-consuming and “unpleasant.” Dying took so long that the anguished victims could not control their bowels. As a result, other camps used stationary killing centres. Chelmno acquired a special reputation for its “efficient” bone-crushing machine (Knochenmühle).

The existence of Chelmno was no secret. An early Jewish escapee in June 1942 got word of the camp to the Polish government-in-exile in London and to Mordecai Chaim Rumkowski, the head of the Judenrat (Judenräte) (German: Jewish Council) in the Łódź ghetto.

From August 1944 to January 1945 the SS burned and obliterated Chelmno's installations and sought to kill all known inmates, in an attempt to erase all traces of the camp. Only two Jewish survivors were found after the war.