Forest appointed the former Wolverhampton Wanderers and Tottenham Hotspur manager after parting ways with Cooper following five defeats in six games which saw them slip to 17th in the table, five points above the relegation zone.
"Nuno joins the club on a two-and-a-half-year contract and will take charge of his first match on Saturday when Forest face Bournemouth at The City Ground," the club said in a statement.
"The new head coach will take charge of his first training session with the squad this morning as they prepare for Saturday's Premier League fixture."
The Portuguese manager spent four seasons at Wolves and helped get them promoted to the Premier League in his first season there.
The former Porto player and manager then took over at Spurs in June 2021 where he won his first three games in charge but then saw his lose five time in seven games before being replaced by Antonio Conte (54).
Nuno was then appointed manager of Saudi Arabian club Al Ittihad and led them to their first league title in 14 years, only to be sacked in November after a run of five league games without a win.
