The Lotus F1 driver beat Le Mans legend Tom Kristensen in the final to claim his first ever triumph in the competition.
The 'Champions of Champions' competition is an individual contest, in which drivers face each other in a 'best of three' knockout format.
Grosjean won the first two races of the final to ensure that he ends 2012 on a high.
The French racer had beaten both Sebastian Vettel and Michael Schumacher in the knockout rounds, ensuring that Team Germany's winning partnership from Saturday failed to add the individual trophy to their vast collection. Vettel and Schumacher have won the Nations Cup for six successive seasons but have yet to triumph in the individual contest.
Grosjean's triumph means that a French driver has won the trophy for a second consecutive season. Sebastien Ogier - who Grosjean teamed up with to finish as runners-up to Germany in the Nations Cup - won the tournament in 2011.