Siegfried Friedrich Heinrich Müller (26 October 1920 – 17 April 1983), referred to as Kongo-Müller or KO-MU, was a German-born soldier and mercenary. Müller was an officer-candidate in the Nazi Germany Wehrmacht in World War II and, after emigrating to Apartheid South Africa, became a mercenary commanding part of 5 Commando in the Congo Crisis. He also trained NATO troops in Europe before moving definitely in South Africa.

Given substantial media coverage by foreign journalists in the Congo, Müller achieved widespread notoriety in West and East Germany in the mid-1960s as a result of his activities in the Congo and his idiosyncratic overt nostalgia for the Nazi era.