Iwakuma's decision was reported Sunday by the Kyodo News agency in Japan. Iwakuma is the second Japanese pitching standout to withdraw from WBC consideration, as Yu Darvish of the Rangers has also said he wouldn't be pitching in the world tournament this spring.
Japan is the two-time defending WBC champion. The 16-team international affair has been held in 2006 and '09 and now is planned for every four years, including the upcoming tournament from March 2-19.
Iwakuma, 31, just signed a two-year, $13 million contract extension that will keep him with the Mariners at least through the 2014 season. He was 9-5 with a 3.16 ERA last season with Seattle, including an 8-4 mark with a 2.65 ERA in 16 starts after moving into the Mariners rotation in July.
Iwakuma didn't address the WBC situation after finalizing his new deal with the Mariners last week in Seattle, but did indicate at that time he was focused on being prepared for his Mariners return after a slow Spring Training last year led to him starting the season in the bullpen.
"I do not see myself changing anything dramatically," he said. "I would like to prepare for the season earlier than the past and come to Spring Training ready to go as opposed to getting prepared in Spring Training."
Iwakuma was one of three pitchers named to the WBC all-tournament team in 2009, after he went 1-1 with a 1.35 ERA in 20 innings over four games for Team Japan, but at that time was pitching for the Rakuten Golden Eagles in Japan's Pacific League.
Iwakuma also was a member of Japan's bronze medal team in the 2004 Olympic Games.
The WBC's schedule will require Major League players who participate to miss the first 4-5 weeks of their team's Spring Training, if their team gets into the championship round in San Francisco on March 17-19.
Mariners ace Felix Hernandez told MLB.com at the end of last season that he plans to compete in the WBC for his native Venezuela and was working with Seattle's training staff on a workout plan to get him ready to pitch a little earlier than normal this offseason.
Hernandez pitched for Venezuela in the 2009 WBC, going 2-0 while not allowing a run in 8 2/3 innings. He then rejoined the Mariners a month into camp and followed up with one of his best MLB seasons, posting a 19-5 record and 2.49 ERA in 34 starts.
Outfielder Michael Saunders is hoping to be invited to compete for Canada, though he said he'll first clear any participation with the Mariners.
Mariners reliever Oliver Perez competed for Mexico in both 2006 and '09 and infielder Alex Liddi would be a logical candidate for Italy, with both those countries having already qualified. Rookie right-hander Erasmo Ramirez might be a pitching candidate for Nicaragua, which is still trying to advance through a qualifying tournament later this month in Panama.