The deal, which sends Aaronson to FC Red Bull Salzburg, is the Union's first outgoing European transfer. According to the Union, it includes "a multi-million-dollar transfer fee, as well as additional performance-based bonuses, for the highest ever transfer fee for a U.S.-based homegrown player."
The Union will also retain a percentage of any future transfer fees for Aaronson.
The Medford teenager will finish the 2020 MLS season with the Union before joining Red Bull Salzburg. He will be eligible to play for his new team when the international transfer window reopens in January 2021.
According to Red Bull Salzburg, Aaronson's contract with the Austrian club runs until 2025.
"Playing in Europe has always been a part of my dream. I've been able to accomplish the first part of my dream, to play for my home city, and am so excited for this continuation of my journey," Aaronson said in a statement.
His first goal was to play on the Philadelphia Union field. Aaronson said his family had tickets to the inaugural Union season and he grew up watching the team from the very beginning.
"I remember hoping one day to be on that field scoring in front of the home crowd," Aaronson said.
It did not take long for him to accomplish that goal. In his club debut last year, Aaronson, then 18, scored against Atlanta United.
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In his second season with the Union, he has recorded seven goals and five assists in 42 matches.
"We are very proud of Brenden and what he has accomplished in such a short amount of time," said Philadelphia Union Principal Owner Jay Sugarman. "I wish Brenden the best at Red Bull Salzburg. I know he'll always have Philadelphia and the Union in his heart and I look forward to seeing what his future holds."
Aaronson is a product of the Union Academy, which he began at age 11. He committed to Indiana University, but then decided to sign a Homegrown Player deal in 2018.
"I am so incredibly appreciative of all my coaches, my family, and the Philadelphia Union fans," Aaronson said.
FC Red Bull Salzburg have won 14 league titles, seven Austrian Cups and three Austrian Supercups.
"Brenden Aaronson is considered one of the greatest talents in the MLS, his dynamism and his presence on the field are extraordinary for a 19-year-old," FC Red Bull Salzburg Sporting Director Christoph Freund said. "Despite his young age, he already has an excellent feeling for making the right decisions on the pitch. With his way of playing football, Brenden is a perfect match for us and we are already looking forward to him."