RSL received allocation money in last week's Expansion Draft for the 22-year old and wished him well as he approaches the next phase of his career.
"Firstly, we would like to thank Chris for his three years of exemplary service on behalf of Real Salt Lake and wish him all the best on this next step in his career," Real Salt Lake General Manager Garth Lagerwey said.
"His professionalism and tremendous talent made this a very difficult move for us to make, however, we felt it was time to do right by Chris and allow him this opportunity to achieve a starting position within Major League Soccer. With the trade also giving our Technical Staff added resources to improve our roster this offseason, we believe the deal is a win for both sides."
Seitz will now have a great chance of playing regular first team football after being second choice to MLS Cup MVP Nick Rimando for the past three seasons and Union coach Peter Nowak is excited about the prospects of the California native.
"We are extremely excited about acquiring Chris Seitz who has been in the U.S. Men's national team goalkeeper pool and with Real Salt Lake the past three seasons," Nowak said. "He's physically imposing and now has a chance to compete to become a starting goalkeeper in MLS and realize his full potential."