More than 1 million people were slaughtered at the camps, most of them Jews.
The wide iron sign — across a gate at the former Nazi death camp in southern Poland where more than 1 million people died during World War II — was removed by being unscrewed on one side and torn off on the other, police spokeswoman Katarzyna Padlo said.
She said the sign — bearing the German words for "Work Sets You Free" — disappeared from the Auschwitz memorial between 3:30 a.m. and 5 a.m.
Police have launched an intensive hunt, with criminal investigators and search dogs sent to the grounds of the vast former death camp, whose barracks, watchtowers and ruins of gas chambers still stand as testament to the atrocities inflicted by Nazi Germany on Jews, Gypsies, and others.
Museum spokesman Pawel Sawicki called the theft a "desecration" and said it was shocking that the tragic history of the site did not stop the thieves.