A day earlier Riley said he expected Chivas, which had already jettisoned seven players from last year's team, to make him the eighth to go and Thursday morning he confirmed that, tweeting from a plane that he was heading to D.C. United.
"Although, I will miss SoCal incredibly, I am excited for the next chapter," he wrote.
Terms of the trade, which is expected to be finalized Thursday night, were not announced, but the Washington Post reported that United will not give up a player in exchange for Riley.
Riley, 30, played for New England, San Jose and Seattle before coming to Chivas in a trade with Montreal during the 2011 MLS expansion draft. With the Galaxy he posted career highs for games and starts (32) as well as minutes played (2,880).
Chivas, which came under the sole control of Mexican businessman Jorge Vergara last August, is remaking its roster under new Coach Jose Luis Sanchez Sola. Since October, Chivas has traded or let go forward Juan Pablo Angel and midfielder Peter Vagenas, who were not re-signed, declined options on defender Danny Califf and forward Alejandro Moreno and midfielders Ben Zemanski and Nick LaBrocca and forward Casey Townsend were traded.
Zemanski was the latest to go, traded to Portland on Wednesday for allocation money and the right of first refusal for former Chivas USA defender Jonathan Bornstein.
Bornstein is currently with Tigres-UANL of the Mexican League, where his playing time has been limited.
Chivas, which is 4-0-1 in preseason play, meets the Colorado Rapids in another exhibition Friday in Las Vegas.