Born in Austria, Kalajdžić is of Serbian descent. He made his debut for Austria on 14 October 2020 in a Nations League game against Romania. He substituted Michael Gregoritsch in the 90th minute. On 25 March 2021, he scored his first two international goals in a 2–2 away draw with Scotland during the 2022 FIFA World Cup qualification. Kalajdžić was included in the squad for Euro 2020, where he scored a goal in a 2–1 loss after extra-time against Italy in the round of 16.