After presenting great progress for his regional team, Watkin was selected in the 36 man squad for the Autumn internationals in 2017. He made his international test debut against Australia on the 11th of November 2017, replacing Owen Williams in the 68th minute. He was recognised by Warren Gatland as an upcoming star and was selected for multiple other squads, including the Grand Slam winning Welsh team in 2019. During the 2019 Six Nations Championship, Watkin scored a try against Italy after a cross-field kick from Gareth Anscombe. Wales' win against Italy was their 11th win in a row, beating their previous record.