The 2021 World Athletics U20 Championships, also known as the World Junior Championships, was an international athletics competition for athletes qualifying as juniors, which was held from 18 to 22 August 2021 at the Moi International Sports Centre in Nairobi, Kenya.

The World Athletics U20 Championships is a biennial world championships for the sport of athletics organised by the World Athletics, contested by athletes in the under-20 athletics age category (19 years old or younger on 31 December in the year of the competition.