Iwamasa was the captain of the Japan team that won the 2003 Summer Universiade held in Daegu, South Korea, where he scored a goal in the final of the tournament. He received the first call-up for Japan national team in 2008 by newly appointed coach Takeshi Okada. On 10 October 2009, he made his international debut in a friendly match against Scotland. He was also one of the final 23 Japan national football players participating in 2010 World Cup although did not play any minute during the tournament. After 2010 World Cup, in January 2011, he was selected Japan for 2011 Asian Cup by new manager Alberto Zaccheroni. At 2011 Asian Cup, he played in 4 matches and Japan won the champions. He played 8 games for Japan until 2011.