The 30-year-old Spanish forward made 19 appearances for West Ham, scoring six goals, following his £4m switch from Arsenal last summer.
Perez had joined Arsenal from La Liga side Deportivo La Coruna for £17m in 2016 but returned on a season-long deal in 2017.
Lucas Perez has continued the summer exodus at West Ham by joining Alaves for £2m
He follows Edimilson Fernandes out the exit door at the London Stadium following his transfer to German side Mainz for an undisclosed fee.
Andy Carroll, Samir Nasri and Adrian were released on free transfers as Hammers boss Manuel Pellegrini has a shake-up of his squad.
West Ham said in a statement: ‘Lucas Perez has joined Spanish Primera Division club Alaves on a permanent deal.
‘West Ham United would like to thank Lucas for his efforts and wish him the best for his future career.
Original article: https://www.dailymail.co.uk/sport/football/article-7101119/Lucas-Perez-leaves-West-Ham-just-one-season-returns-Spain-join-Alaves-2m-deal.html?ns_mchannel=rss&ito=1490&ns_campaign=1490