[2] Beginning his senior career with FC Espoo in 2005, he later played for clubs across Finland, Sweden, Poland, Denmark, and Cyprus.

Internationally, Arajuuri was capped 57 times for the Finland national team and was part of the squad that competed in UEFA Euro 2020.

Arajuuri made his debut on senior level in the ranks of FC Espoo during the 2005 season of the third-tier Kakkonen.

Arajuuri's player agent, Jonas Wirmola, admitted to one of the major newspapers of Åland that Djurgården and several other clubs had shown interest in the young Finnish defender.

[3] Eventually, Arajuuri signed a four-year deal on a Bosman-transfer with the Swedish champions of 2008, Kalmar FF on 24 September 2009.

Kalmar advanced to the third qualifying round, but the team was knocked out by Swiss club BSC Young Boys.

[6] Arajuuri played 12 games, but his strong start was interrupted by a hip injury that kept him sidelined until the end of the season.

In accordance with an agreement reached in August 2013, Arajuuri joined Polish Ekstraklasa club Lech Poznań in January 2014 on a three-year contract.

Lech won the Ekstraklasa and the Polish SuperCup, and the team made it to the UEFA Champions League qualifiers the following season.

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 accordance with a pre-agreement made in July 2016, Arajuuri joined fellow Finn Teemu Pukki at Danish Superliga club Brøndby IF in January 2017.

[16] Arajuuri and Brøndby's 2017–18 season began with the qualifying matches of the Europa League, where they, among others, faced Finnish Veikkausliiga side Vaasan Palloseura.

[19] Brøndby again made it to the Europa League qualifiers, and the team went on to win the Danish Cup by beating Silkeborg IF in the final.

Brøndby once again played in the Europa League qualifiers, where they were eventually knocked out by Genk before reaching the group stage.

[23] In August 2019, Arajuuri moved to Pafos FC, playing in the top-tier Cypriot First Division, for an undisclosed fee.

[29] Arajuuri made his debut in the Finland national under-21 football team in a friendly against Poland in Jyväskylä on 3 June 2008.

[33] He scored his first goal for the national team on 11 October 2015 in a UEFA Euro 2016 qualification match against Northern Ireland.

Along with fellow centre-back Joona Toivio, Arajuuri announced his retirement from international football following 2022 FIFA World Cup qualification match against France on 16 November 2021.