The Burkina Faso international signed a five-year contract with the Ligue 1 club on Monday.
Lyon president Jean-Michel Aulas confirmed that Chelsea will receive a 15% sell-on fee and first refusal should Traore move on.
The club announced their new signing on Twitter: "It is official: Bertrand Traore is Lyonnais! A transfer for €10m (£8.8m) and a five-year contract."
— Olympique Lyonnais (@OL) June 26, 2017
Traore impressed on loan Ajax last season, having scored 13 goals in 39 appearances.
The 21-year managed just 4 goals in 16 appearances for the Blues in three-and-a-half seasons.
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