Sounders paid $4million (628 million naira) as exit fees to the Valencia-based club on Monday.
It's is good business for the club because Martins moved there on a free in September 2012 after a low period in his career playing at Rubin Kazan in Russia where he had fallen out of favour. This also affected his international game with Nigeria.
"Wow, finally I can now announce that I have just signed for Sounders FC. A big step to take in my career. It's a big challenge but I'm ready," Martins tweeted on Monday evening.
"I look forward to this new era in my career...I also look forward to meet my new team mates and the Sounders FC fans, my new family.
"God bless you all, I appreciate all your love, support and well wishes."
He also thanked his fans at Levante.
The striker took a pay cut in order to find more playing time by moving to Spain where he scored seven goals in 21 Liga matches and a further two in three appearances in the Europa League.
He has also reccently been called up to the national team for their 2014 Fifa World Cup qualification match against Kenya this month after failing to make the cut to the 2013 Africa Cup of Nations which the team won.
Martins, 28, is moving to the MLS under the Designated Player Rule (DPR) which allows stars from abroad to be paid higher than the approved MLS salary cap for players.
He is expected to fly to Seattle on Tuesday.
Sounders are playing in the Concacaf Champions League and will be hoping that the Nigerian striker will be able to help them to a successful campaign.