Sergei Aleksandrovich Demagin (Russian: Сергей Александрович Демагин) (born 19 July 1986 in Minsk, Byelorussian SSR) is a Belarusian professional ice hockey forward. He currently plays for HC Neftekhimik Nizhnekamsk of the Kontinental Hockey League. He previously played for Minsk Dynamo. During the 2005–06 season he led the Belarus second league with 61 points in 39 games. In 2008 he signed a contract with the Hartford Wolf Pack of the American Hockey League but never appeared with the team.
Demagin was selected for the Belarus national men's ice hockey team in the 2010 Winter Olympics. He previously represented Belarus at the 2006 World Junior Ice Hockey Championships and the 2007 IIHF World Championship.