The Nigeria international forward has completed his move from Levante and could be available for Saturday's clash against the Portland Timbers.
Martins triggered the buyout clause in his contract with Spanish club Levante earlier in the week, freeing him to sign elsewhere, and he had reportedly been in talks with the Sounders for some time.
"This move is consistent with our promise to the fans." said Majority Owner Joe Roth in a press release. "Our supporters are the best in Major League Soccer and deserve a team that competes each and every year. Obafemi is an electrifying talent who has international experience at the highest level."
After weeks of speculation, Sounders manager Sigi Schmid gave the strongest indication yet that Martins would join the MLS side, telling the Seattle Times on Friday that barring any flight delays, Martins could suit up in rave green as soon as Saturday's rivalry clash against the Portland Timbers.
Martins began his professional career in Italy with Reggiana, before moving to Inter Milan as a teenager. The well-traveled forward later played for Newcastle, Wolfsburg, Rubin Kazan, Birmingham City and most recently Levante, where he scored nine goals in 27 games across all competitions this season.