From 2015 to 2017, Marjanović played the Euroleague Basketball Next Generation Tournament for the FC Barcelona Lassa U18.

[3][4] He made his Adriatic League debut for Mega Bemax on October 4 in a game against the Crvena zvezda mts where he played 5 minutes without scoring.

[11] 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.

[12] He was a member of the Serbian U-16 national basketball team which participate at the 2014 and 2015 FIBA Europe Under-16 Championship.

[13] Marjanović was a member of the Serbian under-20 team that finished 15th at the 2019 FIBA U20 European Championship in Tel Aviv, Israel.