 In August 2017, he signed for Deportivo Alavés after refusing a contract renewal from the Lions.On 2 September 2018, Sainz made his senior debut with the reserves at the age of just 17, playing the last three minutes in a 3–0 Tercera División home win against SD San Pedro.He scored his first senior goal on 7 October in a 2–0 home defeat of SD Beasain, and finished the campaign with five goals in 18 appearances.