The forward is thrilled at the prospect of playing in his homeland while still in the prime of his career following his switch from Tottenham
The United States national team captain was unveiled in front of a packed crowd at the Sounders' CenturyLink Field before their home fixture against Dallas on Saturday after the club had confirmed a transfer believed to be in the region of €7 million.
Dempsey spent just 12 months at White Hart Lane following his switch from Fulham, and he is thrilled at the prospect of returning to his homeland while still in the better years of his career.
"I am very excited about coming back to the MLS and thrilled to be joining Sounders FC," he told the club's official website.
"Having recently played in Seattle with the national team, I experienced the incredible atmosphere and the passion from the supporters first-hand.
"It’s going to be an honor to represent the fans in Seattle.
"This is the league that gave me the opportunity, gave me the chance to be a professional, I'm happy to come back in my prime and make a difference and not (be) coming back when I'm past it."
Dempsey admitted he was preparing for another campaign with Tottenham before he heard of Sounders' interest in signing him.
"Going into the season I thought I was going to be at Tottenham but MLS and Seattle Sounders have moved mountains to get me here and I'm excited to be back in America," the 30-year-old continued.
"The ideal situation is to come in and straight to scoring goals, hopefully it'll be a smooth transition."
Dempsey's previous MLS experience came during a three-season spell with the New England Revolution where he was named rookie of the year in 2004.
He joins a Sounders side seventh in the Western Conference, just two points off the play-off places.