The Spanish centre-back spent last season on loan at the La Liga club.
He made 46 appearances for Newcastle in all competitions after joining from Eibar in 2017.
#NUFC can confirm that Florian Lejeune has completed a permanent move to Alavés for an undisclosed fee.
We thank @lejeune_florian for his efforts, both on and off the pitch, throughout his time at the club and wish him well for the future.
— Newcastle United FC (@NUFC) July 22, 2021