The two-time World Cup veteran joins the MLS Cup holders in a trade that lands the Rapids a draft pick and an international slot.
The Galaxy sent their 2014 second-round draft pick along with an international roster slot to the Rapids for Mastroeni and the rights to midfielder Baggio Husidic. It is the team's second major trade for a U.S. international in less than a month as it acquired Robbie Rogers from the Chicago Fire at the end of May.
Mastroeni is a versatile player who can occupy any position in central midfield or defense and has experience with Galaxy head coach Bruce Arena as the two worked together in the 2002 and 2006 World Cups.
Mastroeni is a long-time Major League Soccer veteran, spending his entire 16-year career in the league with the Miami Fusion and Rapids. The Argentine-born player captained Colorado on its MLS Cup-winning run in 2010.
According to the Galaxy's press release, Mastroeni is in Los Angeles and will begin training with Arena's side on Monday.