Tunisia international Khazri joins on a four-year deal after a long association with Corsican club Bastia.
Khazri broke into the Bastia first-team in 2009 and has impressed with his dynamic performances in recent years.
The 23-year-old scored six goals in 32 appearances last season, yet has now opted to take the next step in his career by moving to the Stade Chaban Delmas.
"The sports project makes me feel immediately envious," Khazri said at his unveiling.
"This club has ambitions and play at the top of the table. It's good for me.
"I have a lot of ambition. I hope I will live up to this new challenge."
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