Jocelyn Gourvennec's men are four points clear of the drop zone in the French top flight with three games remaining, but gave their fans a welcome break from relegation concerns by claiming a straightforward triumph in a repeat of the 2009 showpiece.
Guingamp were victorious in that clash five years ago and repeated the feat as Rennes - who are not yet safe from the drop themselves - missed the chance to end a 43-year wait for a major trophy.
Full-back Jonathan Martins Pereira emphatically put Guingamp ahead late in the first half at the Stade de France and Mustapha Yatabare added a second a minute after the interval, his third goal against Rennes this campaign.
Rennes never looked like mounting a comeback, with Guingamp holding on comfortably to win the trophy for only the second time in their history.
Guingamp made the better start to proceedings and twice spurned opportunities to take the lead in the opening 10 minutes, Christophe Mandanne and Christophe Kerbrat both heading wide in the early stages.
Yatabare then headed off target at the near post from Claudio Beauvue's right-wing cross as Guingamp continued to push for the opener.
Rennes looked to have weathered the storm, but eight minutes before half-time Guingamp did take the lead when Martins Pereira struck in sublime fashion.
The defender found the net with a fierce volley from the edge of the area after Rennes goalkeeper Benoit Costil had failed to clear his lines with a punch from Yatabare's header back across goal.
Guingamp started the second half in the same manner as the first and immediately doubled their lead from the restart through the competition's top scorer Yatabare, who netted his eighth of the tournament with a powerful downward header from Steven Langil's pinpoint left-wing delivery.
Rennes almost crafted an instant response, with only a point-blank save from Mamadou Samassa preventing Jean-Armel Kana-Biyik from pulling one back for Philippe Montanier's men after Guingamp struggled to deal with a corner.
Yet it was Guingamp who were the more threatening throughout the contest, and Gourvennec's side would have extended their advantage had Costil not produced a fine reaction stop to deny Yatabare a second.
Yatabare again went close as he broke free of Rennes' defence before flashing an effort across the face of goal, but his profligacy mattered not, Guingamp easily seeing out a third straight 2-0 win over Montanier's charges to regain the trophy.
