The Spanish club have confirmed that the Nigerian striker triggered the buyout clause from his contract, and Martins announced via Twitter that he had joined the MLS outfit.
Obafemi Martins: "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.
"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..."
Martins joined Newcastle in August 2006 from Inter for €15 million.
He signed a five-year deal. He was paraded in front of the Geordie fans before their UEFA cup tie against FK Ventspils, and inherited the club's cherished number 9 shirt, vacated after the retirement of the club-record goalscorer Alan Shearer in April.
Martins made his debut on 27 August, in the 2–0 defeat to Aston Villa, in which he was stretchered off with a knee injury. It turned out that Martins suffered a severe dead leg and internal bleeding. His unremarkable first few appearances for Newcastle caused manager Glenn Roeder to speak out in his defence.
However, on 17 September, Martins scored his first goal at the Gallowgate End for Newcastle. It was the second goal in a 2–0 victory over West Ham United at Upton Park, and from there his goalscoring form began to improve steadily.
On 14 January, in a 2–3 away win against Tottenham Hotspur, Martins' 20-yard shot rocketed into the net. This strike was clocked by Sky Sports at a speed of 84 mph (135 km/h), making it unofficially the ninth hardest shot ever recorded in football.
Martins ended his first season on Tyneside with 17 goals in 46 games.
His second season was somewhat mixed. With new manager Sam Allardyce preferring to play Michael Owen and Mark Viduka up front, Martins found himself being used as an impact substitute for the first half of the season.
Despite this he managed to score consistently, and when Allardyce was sacked and Kevin Keegan was appointed, Martins found himself being used as part of a three pronged attack along with Owen and Viduka.
His season was interrupted with the African Cup of Nations participation with Nigeria in December.
Martins scored six goals in 12 league games during the first half of the 2008–09 season before a torn hamstring ruled him out for several games.
It was confirmed on 7 January 2009, that Martins would undergo hernia surgery in Germany the next Monday. He was operated on by renowned specialist Dr. Ulrike Muschaweck. He did make it back in time for the last two games of the season, scoring in a relegation six-pointer derby against Middlesbrough.
However, he could not stop Newcastle being relegated to the Championship.
He initially reiterated his desire to stay and help the club return to the Premier League for the 2009–10 season, but he eventually left for VfL Wolfsburg. His last game was on a preseason friendly against Leyton Orient, which Newcastle lost 6–1.
During his three-year spell on Tyneside, Martins amassed 35 goals in all competitions from 104 games.