Wilhelm Oncken (December 19, 1835 - August 11, 1905) was a German historian.
Oncken was born in Heidelberg. The brother of economist August Oncken, he taught history at the University of Gießen.
Oncken was a typical 19th century national liberal, using history as a means of national political education. From the history of ancient Greece, he later turned to the history of Prussia.
Oncken was also editor of the magazine Allgemeine Geschichte in Einzeldarstellungen ("common history in single representations"; 45 volumes, 1876-1893).
Oncken died in Gießen.