The 31-year-old from the Czech Republic could have become a free agent after this season. The Islanders begin the season at Buffalo on Friday night.
"That's great news to start the season," said Islanders coach Ted Nolan. "I'm happy for Marty because he's earned this, but I'm just as happy for our team and our fans because we just locked up one of our key veteran defensemen."
Martinek enters his sixth season with the Islanders -- his only NHL team -- registering 10 goals and 49 assists in 253 regular-season games. He was selected in the 1999 draft and calls Long Island his "second home."
"I never even considered the possibility of playing for another team," he said. "I'm looking forward to this season and many more to come as an Islander."
General manager Garth Snow called Martinek "one of the most underrated defensemen" in the league.
"He is a very steady defenseman in both zones and he plays with a lot of grit," Snow said.