“I was waiting for this moment for a very long time, and I’m excited to play with my new team-mates,” added the 24-year-old, who has scored three times in 19 appearances for his country, including one at the recent Africa Cup of Nations.
“I think, with the quality we have, we can achieve something.”
Palace boss Glasner also said the club are working “24 hours a day” to bring in further new players.
Chairman Steve Parish said Guessand was an “exciting talent” who the club are “confident will flourish”.
“His arrival further bolsters our forward options as we look to push on in the Premier League and in Europe in the second half of the season,” he added.
Guessand was voted Nice’s Player of the Season for the 2024-25 campaign after scoring 12 goals as the French club finished fourth in Ligue 1.