Andrea Raggi came close to a response for Monaco, but saw his curling effort go narrowly wide of the post.
In the second half James Rodriguez tried his luck with a low strike from outside the box, but his effort was comfortably saved by the keeper.
Claudio Beauvue was close to scoring a second for Guingamp, but saw his powerful header cleared off the line.
The visitors threatened soon after as Rodriguez saw his curling free kick saved by Samassa.
In the 85th minute Ladislas Douniama was inches away from finding the back of the net, but saw his powerful long-range drive tipped onto the crossbar by Romero.
The hosts continued to threaten and in the final minute of normal time Yatabare drove into the box before firing an effort wide.
In the first half of extra time Yatabare drilled a volley just past the post, before Moustapha Diallo smashed a shot over the bar.
Monaco struggled to create any chances until the second period of extra time when Rodriguez saw his stinging half-volley fly inches wide of the far post.
The hosts made sure of the win with three minutes of the match left to play, as Giresse broke free down the left before crossing for Yatabare to fire into the bottom corner and send his side through to the final.