[1][2] Kostadinov became the MVP of the 2017 Copa del Rey Junior averaging 15.4 points, 9.6 rebounds and 21.6 PIR per game.
[3] In 2021, he won the Euroleague Basketball Next Generation Tournament with Real Madrid, recording a double-double of 18 points and 12 rebounds in the championship game against Barcelona.
[4][5] On 6 August 2021, Kostadinov signed with Palmer Alma Mediterrànea Palma of the Spanish second division, the LEB Oro.
[7] On 4 August 2022, Kostadinov signed a multi-year contract with Lenovo Tenerife of the Liga ACB and was immediately loaned to Zunder Palencia of the LEB Oro for the 2022–23 season.
[10] Kostadinov helped Bulgaria U16 finish 6th in the 2019 FIBA U16 European Championship Division B, averaging 9.8 PPG, 4.9 RPG and 1.1 APG.