Westbrook, 34, follows golfers Jordan Spieth and Justin Thomas in becoming minor stakeholders in the Whites.
The LA Clippers point guard - listed as the world's 14th highest-paid athlete - was the NBA's most valuable player in 2017 and is a nine-time All-Star.
Joint Leeds owners 49ers Enterprises first invested in the club in 2018.
Their stake grew from 15% to 44%, before agreeing a deal to buy the controlling stake of Andrea Radrizzani in the Championship club - the takeover, though, has not yet been ratified by the English Football League.