In accordance with an agreement reached in August 2013, Arajuuri joined Polish Ekstraklasa club Lech Poznań in January 2014 on a three-year contract. Arajuuri made his debut for Lech Poznań on 5 April 2014 in the home match against Jagiellonia Białystok. He made five appearances at the second half of the 2013–14 season, four of them in the starting line-up. Lech finished second in the Ekstraklasa behind Legia Warsaw who took the championship. Lech made it to the qualifiers for the next season's UEFA Europa League.