Willem "Wim" Hermsen (born September 7, 1947 in Hilversum) is an astrophysicist and a former water polo player from the Netherlands. He finished in seventh position with the Dutch Men's Water Polo Team at the 1972 Summer Olympics in Munich. Two of his brothers, André and Henk, also played water polo on the highest level, and represented the Netherlands in the Summer Olympics, in 1960 and 1964.

Hermsen obtained a PhD from Leiden University and was a professor at the University of Amsterdam from 2004 to 2012, specializing in high-energy astrophysics, and now holds an emeritus position there.

• Dutch Olympic Committee
• UvA album academicum

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