In an entertaining fixture at the City Ground, the Cottagers scored three goals in six minutes to come from behind and win.
The hosts led early on when Taiwo Awoniyi headed in a corner, but were stunned by a second half comeback by the Cottagers.
Joao Palhinha's strike made it 2-1 after Tosin Adarabioyo's goal.
Harrison Reed stroked in thirds.
A wonderful flick from Forest substitute Jesse Lingard set Brennan Johnson free and he teed up Lewis O'Brien as the Cottagers seemed to take control of the match.
Time ran out for Steve Cooper's side to get another goal, but they held on for the win.
It is the second match in a row that Forest have thrown away a lead, the first being a 2-2 draw with Bournemouth.
The Championship runners-up had chances in both halves but were also tested by the hosts.
The two Forest players that came close were Awoniyi and Morgan Gibbs-White.
Six minutes from the end, a ruthless six minutes from Cottagers did the damage as Forest were once again humiliated by the realities of life in the top flight.
Both teams observed a minute's silence prior to the start of the game, before supporters sang the national anthem and applauded in honor of the late Queen Elizabeth II.
The physical edge to the game was the same as it would be in the second tier of English football.
The teams were full of energy and aggression, which made for an interesting spectacle.
Steve Cook was fortunate to receive just a yellow card for a foul on Willian when he was through on goal, while Issa Diop clattered into Forest's Johnson.
As the game grew, the battles became less equal and both teams were in control.
Lingard's flick helped set up O'Brien's goal, which made for a nervous end to the game for the Cottagers.
The visitors settled things down and saw out the win after Forest pushed and pushed for an equalizing goal.
Cooper's side have already shown plenty of threat during their short time in the premier league but they have now conceded 14 goals in their last four defeats which will need to be addressed
The 11 points they have picked up in their first seven matches will be a huge boost to their confidence.
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