Police said the "Arbeit macht frei" (work sets you free) sign that had spanned the gate at the main entrance to the Auschwitz death camp in Poland was stolen early on Friday morning.
The thieves removed the iron sign by unscrewing it on one side of the gate and pulling it off the other side.
Police say they do not have any leads on the identity of the thieves but they are questioning museum guards and checking security camera footage.
Museum spokesman Jaroslaw Mensfelt said it was the first serious case of theft at Auschwitz.