Gilgeous-Alexander, who guided the Thunder to the NBA championship last month after a campaign that saw him win regular season and finals Most Valuable Player awards, is now tied to the club through 2031.
The 26-year-old Canadian point guard has two years remaining on his existing contract with Oklahoma City worth $79.1 million (67.2 million euros).
The four additional seasons of his contract extension will see him receive $62.5 million, $68.6 million, $73.7 million and then $78.7 million in 2030-2031.
The deal gives Gilgeous-Alexander the highest annual salary in the NBA, eclipsing the average $64.3 million salary of the Philadelphia 76ers' Joel Embiid and the $62.8 million earned by the Boston Celtics' Jayson Tatum.
Gilgeous-Alexander was the fulcrum of the Thunder's dazzling 68-win Western Conference season, before inspiring the team to a 4-3 NBA Finals victory over the Indiana Pacers last month.
The Thunder ace averaged a league-high 32.7 points during the regular season and 30.3 points in the NBA Finals on his way to completing his MVP double.
He became only the fourth player in history to win both NBA regular season and finals MVP awards as well as the league's scoring title, putting him into an exclusive club alongside Kareem Abdul-Jabbar, Michael Jordan and Shaquille O'Neal.
News of Gilgeous-Alexander's contract extension came as the NBA's free agency window cranked into top gear after the opening of the window on Monday.