Julien Watrin (born 27 June 1992) is a Belgian sprinter specialising in the 400 metres. In 2023, he won his first individual major senior medal, with silver at the European Indoor Championships. Watrin earned several major medals with national men's 4 × 400 m relays, including bronzes at the 2019 and 2022 World Athletics Championships.

He is the Belgian record holder for the 400 m hurdles, Belgian indoor record holder for the 400 m, and won several national titles.

He was diagnosed with testicular cancer in September 2023 which prevented him from participating in the 2024 Summer Olympics in Paris, France. After successful treatment, he made his return on the international stage at the 2025 European Athletics Indoor Championships in Apeldoorn, The Netherlands, winning a silver medal as a member of Belgium's 4 × 400 m mixed relay team and a bronze medal as a member of Belgium's men 4 × 400 m relay team.

In March 2026, he was selected for the relays at the 2026 World Athletics Indoor Championships in Poland. On the second day of competition, on 21 March, he won the gold medal in the first ever indoor mixed 4 × 400 metres relay running alongside Ilana Hanssens, Jonathan Sacoor and Helena Ponette Their time of 3:15.60 was the fastest short track performance recorded for the event. One day later, on 22 March, he won a silver medal in the men's 4 × 400 metres relay running alongside Christian Iguacel, Jonathan Sacoor and Alexander Doom.