West Ham United hired Nuno Espirito Santo on Saturday after sacking Graham Potter as manager following a dismal start to the season.
West Ham said Nuno has signed a three-year contract and will take charge when the club face Everton in the Premier League on Monday.
Potter, 50, took over at West Ham in January, replacing former Spain and Real Madrid coach Julen Lopetegui, but was unable to oversee a change in fortune at the London Stadium.
West Ham have won just six of 25 games under Potter, losing 14, and they went into this weekend's Premier League fixture list one place above the bottom spot in nineteenth position after losing four of five games this season.
Having conceded 13 goals in five games so far, West Ham have the worst defensive record in the top flight and supporters have vented their anger at both Potter and the club hierarchy during recent games.
In a statement issued Saturday morning, West Ham confirmed the decision to part company with Potter.
"West Ham United can confirm that head coach Graham Potter has left the club," the statement said.
"Results and performances over the course of the second half of last season and the start of the 2025-26 season have not matched expectations, and the board of directors believe that a change is necessary in order to help improve the team's position in the Premier League as soon as possible.
"The club can confirm that assistant coach Bruno Saltor, first-team coaches Billy Reid and Narcis Pelach, lead goalkeeper coach Casper Ankergren, and goalkeeper coach Linus Kandolin have also left with immediate effect.
"The board would like to thank Graham and his coaching staff for their hard work during their time with the Hammers and wish them every success for the future.
"The process of appointing a replacement is under way. The club will be making no further comment at this time."
Former Wolves and FC Porto coach Nuno guided Forest to European qualification last season, but he was dismissed by club owner Evangelos Marinakis last month and replaced by former Tottenham manager Ange Postecoglou at the City Ground.
Potter issued a statement later on Saturday, saying he was disappointed to be let go.
"I am incredibly disappointed to be leaving West Ham United football club, particularly without being able to achieve what we set out to achieve at the start of our journey in East London," Potter said in a statement via the League Managers' Association (LMA).
"I do however fully acknowledge that the results have just not been good enough up to now."