Johann von Werth, also Jan von Werth or in French Jean de Werth, was a German general of cavalry in the Thirty Years' War.

The Duchy of Jülich comprised a state within the Holy Roman Empire from the 11th to the 18th centuries. The duchy lay west of the Rhine river and was bordered by the Electorate of Cologne to the east and the Duchy of Limburg to the west. It had territories on both sides of the river Rur, around its capital Jülich – the former Roman Iuliacum – in the lower Rhineland. The duchy amalgamated with the County of Berg beyond the Rhine in 1423, and from then on also became known as Jülich-Berg. Later it became part of the United Duchies of Jülich-Cleves-Berg.