The 32-year-old was initially handed a yellow card for dissent by on-field referee Anthony Taylor after challenging a decision from VAR to award Forest a penalty midway through the second half, with the score at 3-1 at the City Ground.
After reviewing the coming-together between Jack Hinshelwood and Callum Hudson-Odoi at the pitchside monitor, Taylor awarded the spot kick, much to the disgust of Dunk who was then shown a straight red card for foul and abusive language.
Morgan Gibbs-White converted the resulting penalty kick, but Brighton held on to claim all three points with 10 men, ending their six-game winless league run in the process.
A straight red card for such an offense carries a two-match suspension, meaning Dunk will now be forced to miss top-flight fixtures against Chelsea at Stamford Bridge on December 3, and at home to Brentford three days later.
Dunk's dismissal for foul and abusive language is the first during a game in the Premier League since January 2008 when former Newcastle United man Alan Smith was shown red in a 6-0 defeat to Manchester United.
Over the years, there have been rare occasions when players have been sent off after the full-time whistle for dissent, including ex-Brighton striker Neal Maupay against Wolverhampton Wanderers.
Lee Cattermole was sent off for the same offense after the match against rivals Newcastle in March 2012, as was former Wolves midfielder Matheus Nunes when on the substitutes' bench against Leeds United in March this year.
Dunk's red card was the fifth of his 13-year senior career and his first since February 2022 away against Man United.
The England international had played the full 90 minutes in 11 of Brighton's first 12 Premier League fixtures before the win at Forest - missing the 1-1 draw with Sheffield United before the international break due to injury.
Dunk is not the only absentee for Brighton, as Roberto De Zerbi is also having to deal with a number of injury concerns including Ansu Fati and Tariq Lamptey, who were both forced off in the first half at the City Ground with muscle problems.
Kaoru Mitoma, James Milner, Adam Webster, Igor Julio, Pervis Estupinan, Solly March, Danny Welbeck and Julio Enciso are all currently sidelined through injury, while Mahmoud Dahoud served the first of a three-game domestic ban against Forest.
The absence of the suspended Dunk and possible absences of injured duo Webster and Igor could see Joel Veltman partner Jan Paul van Hecke at centre-back against Chelsea, with Pascal Gross potentially filling in at either right-back or left-back
Both Dunk and Dahoud are available to represent Brighton in Thursday's Europa League group-stage clash with AEK Athens in Greece.