Marjanović was a member of the Serbian U-18 national basketball team that won the gold medal at the 2017 FIBA Europe Under-18 Championship. Over seven tournament games, he averaged 6 points, 3 rebounds and 0.9 assists per game. He was a member of the Serbian U-16 national basketball team which participate at the 2014 and 2015 FIBA Europe Under-16 Championship. Over nine 2015 tournament games, he averaged 14.1 points, 3.2 rebounds and 1.1 assists per game. Marjanović was a member of the Serbian under-20 team that finished 15th at the 2019 FIBA U20 European Championship in Tel Aviv, Israel. Over seven tournament games, he averaged 7.1 points, 1.9 rebounds, and 0.9 assists per game.