On 7 March 2008, when Kumagai was 17 years old, she debuted for the Japan national team against Canada. In August, Kumagai was selected for the Japan U-20 national team at the 2008 U-20 World Cup. In 2010, she played for the U-20 team as captain during the 2010 U-20 World Cup. In 2011, she was part of Japan's World Cup-winning team, scoring the winning penalty in the final against the United States. She was also in the squad at the 2012 Summer Olympics and the 2015 World Cup. Japan came second at both competitions. In January 2017, she was named Japan's captain by manager Asako Takakura. In 2018, Japan won the 2018 Asian Cup. She has played more than 100 games for Japan. On 10 November 2019, Kumagai scored her first ever goal in a friendly match for Japan in a 2–0 win against South Africa.