Lech and Arajuuri's 2015–16 season began with European qualification. After being knocked out of the UEFA Champions League qualifiers, the team qualified for the Europa League group stage. Arajuuri made his debut in the Europa League on 17 September 2015 in the home match against Belenenses. However, the team finished third in the group behind FC Basel and Fiorentina. Arajuuri continued to appear as a key player of the team and played a total of 28 league matches, of which 27 were in the starting line-up. In addition, Arajuuri made three Europa League appearances. That season, Lech finished seventh in the Ekstraklasa.