The 36-year-old attacker still had 12 months to run on his contract with the side from the principality but they agreed to part company over the weekend
The veteran attacker, who was capped 17 times by his country, signed a one-year contract with Les Merlus after successfully completing a medical on Tuesday.
"I am very happy to arrive at FC Lorient," Giuly enthused. "This is a club that suits me because there is a lot of enjoyment in their play, something that has been instilled in them by their great coach, Christian Gourcuff.
"I know that they are counting on me on the field and in the dressing room, and I will give everything to have a great adventure with FCL."
Giuly, who won the Champions League with Barcelona, still had a year to run on his contract with Monaco, whom he rejoined last summer, but the player and the club parted company by mutual consent over the weekend.