The decision follows a breakdown in his relationship with club owner Evangelos Marinakis.
Nuno had only signed a new contract in June after guiding Forest to Europa League qualification last season, their first European appearance since 1995.
He had previously questioned the club's summer transfer business and his long-term future at the City Ground.
Nottingham Forest's statement thanked Nuno for his contribution, particularly his role in their 2024/25 season.