"Coming into the season I thought I was going to start with Tottenham but the MLS and Seattle Sounders have moved mountains to get me here," the 30-year-old Dempsey told the Sounders website (www.soundersfc.com).
"I'm happy to come back in my prime and be able to make a difference and not when I'm past it."
Premier League Tottenham said on Saturday they had reached an agreement with MLS for Dempsey and he was presented by Sounders before their match against FC Dallas.
"I'm excited about coming up here," added Dempsey, who scored 12 goals in 43 appearances for Tottenham after joining from Fulham.
Dempsey, who has played 99 times for the United States and is captain of the national side, played for the New England Revolution for two seasons before joining Fulham in January 2007.
He leaves Tottenham following the arrival of Spain's Roberto Soldado from Valencia and Brazilian international Paulinho from Corinthians.
Seattle regularly attract 40,000-plus fans to home matches and the move represents a major coup for the club, where Dempsey will join former Inter Milan and Newcastle United striker Obafemi Martins.
The MLS season is halfway through and Seattle's 3-0 win over Dallas took their unbeaten home sequence to nine.