Jan Marcus Posthuma (born 11 June 1963, in Dokkum, Friesland) is a retired volleyball player from the Netherlands, who represented his native country in three consecutive Summer Olympics, starting in 1988 in Seoul, South Korea.
After having won the silver medal in 1992, Posthuma won the gold medal in Atlanta, Georgia (1996) with the Dutch Men's Volleyball National Team by defeating Italy in the final (3–2).