Jonathan ("Jon") Scott Sieben, OAM (born 24 August 1966) is an Australian former butterfly swimmer active in the 1980s, who won gold in the 200 m butterfly at the 1984 Los Angeles Olympics.
Hailing from Brisbane, Queensland, Sieben was coached by Laurie Lawrence, and made his debut at the young age of 15 at the Brisbane1982 Commonwealth Games, where he captured a bronze in the 200 m butterfly, as well as gold in the medley relay. Sieben and Lawrence continued their preparation for Los Angeles, but were given little chance. Sieben was known to his friends as The Shrimp, as he stood just 173 cm, in contrast to the dominant swimmer of the time, West Germany's Michael Groß, the world record holder, known as The albatross with his 200 cm frame and 225 cm wingspan. He also faced the 100 m butterfly world record holder, the United States' Pablo Morales. Sieben swam in the wake of Groß and Morales in the first 150 m, before storming home in the last 50 m to claim the gold medal in a world record time of 1 m 57.04 s, more than four seconds faster than he had ever swum before. He also collected a bronze medal after swimming in the preliminaries of the medley relay. He was named the Young Australian of the Year in 1984.
By the 1988 Seoul Olympics, Sieben's ability had waned, and the selectors did not choose him for the 200 m butterfly. He finished fourth in the 100 m butterfly. He continued onto the 1992 Barcelona Olympics, but failed to reach the final of the 100m butterfly individual event. He formed part of Australia's 4X100m medley relay team that finished 7th in the final, in a national record time of 3:42.65.