Pereira, a former Wolves boss, arrives with Forest precariously placed in 17th in the table, three points above the drop zone.
He previously worked under Forest owner Evangelos Marinakis at Olympiacos, guiding them to a league and cup double in 2014/15.
Pereira replaces Sean Dyche, who was sacked after a 0-0 draw with Wolves, following the earlier departures of Ange Postecoglou and Nuno Espirito Santo this season.
His first matches in charge will be a Europa League knockout play-off against Fenerbahce, followed by a Premier League fixture against Liverpool.
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