Aaronson moved to Leeds United from FC Salzburg last year in a transfer worth roughly $30 million (£23.6m), but after Leeds were relegated from the English Premier League this season, he sought a move elsewhere. Union Berlin finished fourth this year in the Bundesliga and qualified for the UEFA Champions League.
The loan was first reported by The Athletic.
In his lone Premier League season, Aaronson appeared in 36 of 38 matches, scoring once and registering three assists. He initially moved to Leeds to reunite with manager Jesse Marsch, the fellow American who also coached him at Salzburg. Marsch was fired in February.
Aaronson debuted for the USMNT in 2020 and established himself as a key player during World Cup qualifying, playing in midfield and as a winger. He came off the bench in all four U.S. matches at the World Cup, playing 108 minutes.
The 22-year-old spent two years with the Philadelphia Union in Major League Soccer before making the jump to Salzburg in 2020.