On February 18, 2006, when Iwashimizu was 19 years old, she debuted for the Japan national team against Russia. She has played in the 2007, 2011 and 2015 World Cups, winning the 2011 World Cup and winning a silver medal at the 2012 Summer Olympics. In the final minute of extra time in the 2011 World Cup Final, she was sent off after receiving a red card for slide tackling Alex Morgan. Japan would go on to win the match in the penalty shootout that followed.