The 30-year-old said he was delighted to commit to a club he believes can build on recent success.
"I've settled down here and I'm excited to commit to the club for the next few years," Martins told BBC Sport.
"The fans are amazing, the club is awesome and together we are building something massive here."
He added: "My ambition is to win more trophies here and continue to give my best."
The former Newcastle United and Birmingham City forward joined Sounders in March 2013, and scored eight goals and created four assists in his MLS debut campaign.