Wilhelmus Antonius Sassen (born 16 April 1918 – died 2002) was a Dutch collaborator, Nazi journalist and a member of the Waffen-SS, where he had the rank of Untersturmführer corresponding to 2nd Lieutenant. He became widely known around 1960 as the interviewer of Adolf Eichmann.
Willem Sassen was born in Geertruidenberg, Netherlands. He grew up in a traditional Roman Catholic family in North Brabant and attended a gymnasium in Neerbosch near Nijmegen and in Breda. His father was impressed by the fascist ideas of the Zwart Front (literally 'Black Front'). Sassen decided not to become a priest but to study law in Leuven and Gent instead. As a student, Sassen became a member of the German-Flemish workers' group DeVlag. When Sassen visited the 1936 Summer Olympics in Berlin, his admiration for Hitler and Nazi Germany grew. Because of Sassen's pro-Nazi political activities, the authorities expelled him from Belgium, so he could not finish his law studies.
In 1938 Sassen became a recruit in the Dutch army and was trained as an artillerist. When Nazi Germany invaded the Netherlands on 10 May 1940, Sassen was a member of the 7th Field Artillery regiment and was taken prisoner by the Germans for a short time.