His dismissal came immediately after Forest's 3-0 defeat to Chelsea on Saturday, a match where owner Evangelos Marinakis left his seat before full-time.
Postecoglou failed to secure a single win during his tenure, making him the first Forest manager in the club's history to not win any of his first eight games.
A brief club statement confirmed his departure, citing a “series of disappointing results and performances”.
Nottingham Forest, currently 17th in the Premier League, is now searching for their third manager of the season, with Sean Dyche reportedly a potential candidate.