The Spaniard returns to his homeland after a single season with West Ham United, during which he scored six goals in 19 appearances.
Lucas became the first Hammers substitute in 19 years to score twice after being introduced from the bench into a Premier League match when he netted a brace in the 3-1 win over Cardiff City in December 2018.
The 30-year old who joined the Hammers from Arsenal for £4m and was reported to earn £75,000 per week has joined Alaves on a three-year contract.
He follows Edimilson Fernandes out the exit door at West Ham following his £8m transfer to German side Mainz earlier in the day.
Andy Carroll, Samir Nasri, Toni Martinez and Adrian have been released by West Ham manager Manuel Pellegrini continues a major shake-up of his squad.