Lee Jae-sung was born on 10 August 1992 in Ulsan, South Korea. After developing through Korea University, he joined Jeonbuk Hyundai Motors in 2014 and quickly became a central figure because of his intelligence, work rate and ability to appear in the right pockets rather than relying on raw speed.
At Jeonbuk he won multiple K League titles and the 2016 AFC Champions League, establishing himself as one of the best domestic midfielders in Asia. In 2018 he moved to Holstein Kiel in Germany's second division, where his pressing and combination play translated smoothly to European football.
Mainz 05 signed him in 2021, and he has remained a key Bundesliga player into 2026, signing a contract extension in February 2026. At Mainz he has been used as an attacking midfielder, second striker and wide midfielder, making his off-ball intelligence central to the team's pressing structure.
For South Korea he debuted in 2015 and has played at the 2018 and 2022 World Cups, the 2019 and 2023 AFC Asian Cups and the 2026 qualifying cycle. He is one of the senior connectors around Son Heung-min, Hwang Hee-chan and the younger creative players.
At around 1.80 m, Lee is a left-footed attacking midfielder whose pressing, timing and one-touch combinations define his game. He is often compared to Park Ji-sung for intelligence and industry, though Lee is more of a creative connector than a pure ball-winning runner.