The "Arbeit macht frei" (work sets you free) sign was stolen by a gang of Polish thieves acting at the behest of a Swedish far-right-winger.
Technicians unveiled the restored sign in the laboratory of the camp museum.
More than one million people, mostly Jews from across Europe, were murdered by the Nazis at the camp.
Most of the work to restore the damaged sign was done on site, but a master blacksmith welded it back together.
The museum's director, Piotr Cywinski, said the sign would probably form part of a new exhibition.
A copy of the original has been placed above the entrance gate.
Thieves had cut up the black wrought-iron sign into its three constituent words in order to fit it into their getaway car after taking it down from the main gate.
Following a nationwide search, police found the sign a few days later in a rural area hundreds of kilometres away.
Five Polish men have been convicted of carrying out the theft on behalf of a Swedish man, Anders Hoegstroem, who helped found the far-right National Socialist Front party in Sweden in 1994.
He is serving a prison sentence in his homeland following his conviction in Poland.
Museum officials at the former Auschwitz concentration camp in Poland have restored the metal entrance sign damaged in a theft 17 months ago.