Sun Jiajun (born 1 August 2000) is a Chinese swimmer. He won the gold medal in the boys' 100-metre breaststroke and 4 × 100-metre mixed medley relay events at the 2018 Summer Youth Olympics held in Buenos Aires, Argentina. He also won the silver medal in the boys' 50-metre breaststroke and 4 × 100-metre medley relay categories in the same event. In 2021, he qualified to represent China in the men's 4 × 100-metre medley relay event at the 2020 Summer Olympics held in Tokyo, Japan.