The France Under-21 international had been linked with Premier League clubs, but has instead sealed a move to LaLiga, signing a five-year deal.
Diakhaby's contract at Valencia includes a €100m release clause, while he will be presented at his new club on Friday.
The centre-back, 21, started just eight Ligue 1 games in 2017-18, having featured in Lyon's starting XI 22 times in the previous campaign.
28 June 2018