21-year-old midfielder Aaronson will move to the Austrian giants in January 2021 after the clubs agreed terms.
Wir freuen uns auf dich, Brenden #Aaronson! 🙌
— FC Red Bull Salzburg (@RedBullSalzburg) October 16, 2020
Salzburg will pay an initial $6m (€5.1m) with a further $3m (€2.6m) to follow in bonuses.
Union will also receive a percentage of any future fee for Aaronson, who has signed a five-year deal with the Red Bull club.
It is the largest ever fee paid for a U.S.-based homegrown player and Union’s first export to Europe.
Aaronson broke into Union’s first team in 2019 and has started all of the club’s 18 matches this year.
His performances have impressed onlookers with a return of seven goals and seven assists in his career.
Aaronson won his first cap for the senior U.S. men’s national team earlier this year.
Aaronson’s potential is such that if all add-ons are met, Salzburg will be paying more for him than they did for Erling Haaland in 2019.