In May 2012, Krafth was called up to the Sweden U21 squad for the first time. Krafth became the fifth youngest person ever to play in the Sweden U21 team, being 17 years and 309 days old when he played in Sweden U21s' match against Malta on 6 June 2012. Krafth then kept three consecutive clean sheets between 5 June 2014 and 5 September 2014 against Iceland U21, Slovakia U21 and Greece U21, all of them were wins. However during a 2–2 draw against Denmark U21 on 11 June 2015, he suffered a back injury that kept saw him miss the UEFA European Under-21 Championship and Sweden U21 went on to win the tournament. A year later on 3 June 2016 against Georgia U21, Krafth scored the U21 side's second goal of the game, in a 3–2 win, which turns out to be his last appearance for Sweden U21. He went on to make sixteen appearances and scoring once for the U21 side.