While David Beckham might be looking to move back to Europe - at least for a short-term loan - Ljungberg, 31, made a two-year commitment to play for Seattle, signing a contract reported to be worth US $5 million per year.
The former Arsenal and Sweden star has been looking for a club after he terminated his deal with West Ham by mutual consent this summer.
"I'm really happy to be here," Ljungberg said. "I feel so welcomed by everybody."
Ljungberg joins former Tottenham and Borussia Monchengladbach goalkeeper Kasey Keller - a Seattle native - as well as midfielder Sanna Nyassi and striker Sebastien Le Toux as the only players on the club's roster at present, although an expansion draft is due on November 26.
The mooted deal has sparked fresh discussion about the standard of MLS and its ability to attract and retain high-quality players.
However, Ljungberg said he was not concerned and actually made the decision despite friends telling him to stay in Europe.
"Maybe the league is not that great, but I think that is almost proven wrong with David going to Milan - if he can crack it at Milan then he's still doing well and that shows MLS has a good level if he can do that," Ljungberg said.
"For me personally, I'm coming here because I want to play football and I see this as a great experience and I want to make it a better league and develop it.
"I felt if I was going to do this as a sincere thing, I should go now. I shouldn't go in three years when I'm past my peak."
Sounders co-owner and Hollywood director Joe Roth compared the signing of Ljungberg to the deal that took Beckham to the Galaxy in the summer of 2007.
"I would say in terms of significance to the league, he is certainly one of the two significant European players to come to this league so far," Roth said.
The Sounders are owned by Roth and Microsoft co-founder Paul Allen, as well as a number of minority owners including comedian Drew Carey.
More than 17,000 season tickets have been sold for the 2009 season, which already puts the Sounders second in MLS.