A 3-0 defeat at home to Chelsea left Postecoglou winless in eight games since taking charge after Forest's axing of Nuno Espirito Santo.
In a short statement, 20 minutes after the final whistle, Forest announced:
"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.
"The club will make no further comment at this time."
Postecoglou was in charge for eight games at Nottingham Forest.
Six of those ended in defeat, including an early Carabao Cup exit at Championship club Swansea City, when Forest led 2-1 in the 92nd minute before conceding two in four minutes to lose 3-2.
The other two games - away at Burnley (Premier League) and Real Betis (Europa League) ended in draws.