Forest were thrashed 3-0 at home to Chelsea on Saturday.
After 20 minutes of the full-time whistle, the club announced Postecoglou had been dismissed.
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“Nottingham Forest Football Club can confirm that after a series of disappointing results and performances, Ange Postecoglou has been relieved of his duties as head coach with immediate effect,” read the statement.
“The club will make no further comment at this time.”
Postecoglou, 60, signed a two-year deal at the City Ground on September 9, following the departure of Nuno Espirito Santo.
But he lost six times and managed just two draws during his 39-day tenure.
