The 30-year-old forward is returning to the United States after six seasons in England, and just 12 months after arriving at White Hart Lane
The United States captain only arrived at White Hart Lane last summer, from Fulham, but he failed to nail down a regular starting berth under Andre Villas-Boas, so has now decided to return home after six years plying his trade in England.
"This is a historic day for Sounders FC and our fans," Seattle Majority Owner Joe Roth said in a statement.
"Clint is a world-class soccer player who can take this club to a higher level. We have been consistent in our message to the fans that we will spare no expense to win trophies."
Seatle owner and general manager Adrian Hanauer is also enthused by his club's acquisition of a player who has netted 35 times in 99 appearances for his country.
"In doing something of this magnitude we had to make sure we had the right player. Clint Dempsey fits that mold," the Sounders supremo stated.
"Clint is a proven winner, a leader and a competitor who we believe will raise the bar for this organisation on and off the field."
Dempsey began his career with the New England Revolution in 2004 before moving to Fulham three years later.