Guingamp, who are battling relegation from Ligue 1, triumphed with goals by Jonathan Martins Pereira and Mustapaha Yatabare either side of halftime.
Five years after beating Rennes 2-1 in the final, Guingamp, under former Nantes midfielder Jocelyn Gourvennec, lifted the trophy again with a spririted performance in a festive atmosphere.
Martins Pereira opened the scoring when he fired into the top corner eight minutes before the break after Christophe Mandanne's header was parried into his path by Benoit Costil.
Yatabare doubled the lead a minute into the second half when he headed home from Steeven Langil's perfect cross and Rennes, who last won the French Cup 43 years ago, were beaten.
Guingamp's next challenge will be to retain their top flight status. They are four points above the drop zone in 16th place in Ligue 1 with three games left and face difficult matches at Monaco and Olympique Marseille and then home to Toulouse.
- Sports & Recreation
- Christophe Mandanne
- Jocelyn Gourvennec