"I felt the club's confidence in me from the first discussions, and I'm excited to get started."
Truffert made his first-team debut for Rennes in 2020 and made 212 appearances for the club where he became club captain.
He progressed through France's youth levels before making his senior debut in 2022.
"Adrien is a player that we've admired for some time and he brings significant experience to the squad despite still being just 23," said Bournemouth president of football operations Tiago Pinto.
"We're excited to work with Adrien and are thrilled to have a player of such quality join us."