Reus, 35, was a free agent after leaving Dortmund at the end of last season, playing his final game in the Champions League final defeat against Real Madrid.
It dropped the curtain on a 12-year stint at the German club, where he played 429 games across all competitions and scored 170 goals.
He also won 48 caps for Germany and appeared at Euro 2012 and the 2018 World Cup.
"Marco Reus is a world-class player who has excelled at the highest level of the sport," said LA Galaxy general manager Will Kuntz.
"For Marco, a two-time Bundesliga Footballer of the Year, to join the Galaxy after playing his entire career in Germany is a testament to the project we are continuing to build at the club.
"We look forward to Marco's contributions to the LA Galaxy as we enter the critical final stretch of the 2024 MLS season and beyond."
Galaxy currently top the Western Conference in the MLS, and lie second behind Inter Miami in the overall table. Their next game is at home to Atlanta United on August 24.
