Cooper was appointed head coach at Championship club Nottingham Forest on 21 September 2021 on a two-year contract. At this point, Nottingham Forest were bottom of the division. Cooper improved Forest's style of play by encouraging them to be confident with the ball and attack in greater numbers. He also improved the mentality of the players – fostering a greater sense of togetherness, and instilling a 'big club mentality'. On 29 May 2022, Cooper led Nottingham Forest to promotion to the Premier League, after a 1–0 win over Huddersfield Town in the Championship play-off final at Wembley Stadium.
