The La Liga club announced the news on Friday. The Oklahoma City Thunder star guard is a 2025 NBA champion and a two-time league Most Valuable Player (MVP).
"It is a club with a long-standing tradition at the pinnacle of European football, and I am grateful for the opportunity to be part of its future," Gilgeous-Alexander said.
Atlético, whose squad includes Slovak defender Dávid Hancko, did not disclose the size of his ownership stake. According to the club's statement, the partnership is founded on shared values of ambition, the pursuit of excellence, and the conviction that every success must be earned.
"His investment reflects a commitment to the club's long-term vision and its international growth," Atlético stated.
Gilgeous-Alexander follows the path of several other high-profile North American sports stars. NBA icon LeBron James became a partner in Fenway Sports Group in 2021, whose portfolio includes Premier League side Liverpool and MLB's Boston Red Sox.
NFL legend Tom Brady also owns a minority stake in English Championship club Birmingham City. Atlético currently sit second in the Spanish top flight and travel to face Deportivo Alavés on Saturday, AP reported.