The 24-year-old winger was signed with Targeted Allocation Money and has agreed to a three-year deal with the club that runs through the 2023 season.
"Samuel is an attack-minded, talented player who will strengthen our roster," general manager Dennis te Kloese said in a statement. "He has demonstrated to be an outstanding and talented player in a very respected and competitive league.
"We are confident he will show his potential and are excited to watch him grow even more and have him become an important part of our roster. We look forward to him joining our squad and integrating Samuel to the team."
Grandsir scored 12 goals while notching 11 assists in 131 appearances for Troyes, Monaco, Strasbourg and Brest across all competitions, including UEFA Champions League and Ligue 1. Most recently he had five goals and an assist for Stade Brestois 29 while on loan since 2019.