Nicknamed 'The Farmers', Guingamp dominated from the outset to the delight of their fans, hundreds of whom had travelled the 480km to Paris in a convoy of tractors.
Jonathan Martins Pereira got the opener after 36 minutes when he latched onto a Christophe Mandanne header to catapult into the net. And Malian international Mustapha Yatabare claimed the second just after the break when he slipped through the Rennes defence to shock goalkeeper Benoit Costil with a close range header for his eighth goal in the competition.
It was the third victory this season for Guingamp over their fellow Bretons whom they beat twice in the league, and another woeful end for Rennes who also lost the 2009 Coupe de France final against the same side, who were then in the second tier.
Rennes were also beaten by Saint-Etienne in last season's Coupe de la Ligue final, leaving them still without a major title since beating Lyon to lift the Cup back in 1971.
Guingamp - the club where Didier Drogba shot to prominence - has a population of little more than 7,000. Both clubs remain in danger of relegation to the French Ligue 2 with just three games left in the league campaign.