The Wales international forward has agreed a deal with the Club that will run until 2029. He will wear the number 22 shirt.
He spent a successful loan spell at Lincoln City in 2020/21, where his 13 goals helped the Imps reach the League One Play-Off Final - while his form also earned him a first senior cap for Wales, coming against the USA in November, 2020.
Back at the City Ground, Brennan starred as Forest gained promotion from the Championship into the Premier League in 2021/22. He scored 19 goals in all competitions, including 16 in the league and was on target in both legs of the semi-finals against Sheffield United as Forest returned to the top flight via the play-offs. His performances were recognised by the EFL - he was EFL Young Player of the Month in September, Championship Player of the Month for April and the Championship's Young Player of the Season.
Brennan was ever-present in his first season in the Premier League, contributing eight goals in 38 appearances and another two as Forest reached the semi-finals of the Carabao Cup.
Capped across all youth levels by Wales, Brennan already has 20 senior caps to his name and was part of Rob Page's squad for the World Cup last year.