Riley made over 200 appearances for the Sounders in the MLS, and will now be a part of the Cascadia team.
"A friend involved with the set-up told me about the opportunity and thought I'd be intrigued, and I certainly was," Riley told 'Sky Sports'.
"I had conversations with the internal structure of Cascadia, and it was something where the timing just felt right.
"I think the mission of CONIFA is fantastic - to bridge cultures and groups through the beautiful game. The chance to represent Cascadia in their first World Football Cup is something really special.
"You've got the Sounders, the Whitecaps, and the Timbers and that forms part of the Cascadian rivalry and culture.
"It's also a stunning place to live, and to raise a family. To be able to unify those three cities that have such a deep-rooted soccer history is fantastic.