Security officials noticed this morning that the gate measuring 190 by 95 centimeters (75 by 37 inches) set into a larger iron gate was missing, police said in a statement.
Whoever stole it during the night would have had to climb over another gate to reach it, they added.
Police said they found nothing in the immediate vicinity of the camp and appealed for anyone who noticed any suspicious people or vehicles to come forward.
Memorial director Gabriele Hammermann condemned the theft of the gate, which she described as "the central symbol for the prisoners' ordeal," news agency dpa reported.
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She said a private security service supervises the site but officials had decided against surveillance of the former camp with video cameras because they didn't want to turn it into a "maximum-security unit." That decision may now have to be reviewed, she added.
A Swedish man who had a neo-Nazi past was found guilty of instigating that theft and jailed in his homeland. Five Poles also were convicted of involvement and imprisoned.