The 24-year-old was brought on board by coach Abelardo Fernandez Antuna in a bid to improve the club's performance in the upcoming campaign after they beat the drop on final day last season.
“I know I’m going to face massive competition at Deportivo Alaves because they have got good and wonderful players," Twumasi told 442gh.
"But it's up to me to justify myself in the team and it all comes down to hard work and dedication ahead of the new season so I'm ready for it,” he added.
Twumasi, who has three international caps with the Black Stars, is the second Ghanaian at the Mendizorrotza after Wakaso Mubarak.
At Astana, he scored 18 goals and provided three assists in 29 appearances.