Diodurum, Archaeological site in Jouars-Pontchartrain, France.

Diodurum is an archaeological site in Jouars-Pontchartrain, Yvelines, where excavations between 1994 and 1998 uncovered necropolises, temples, sanctuaries, dwellings and the ruins of an ancient theatre. A long ruined stone wall with patches of red brick and climbing plants stands on the site. Since 2003 research has focused on the Cistercian buildings and a quarter of the ancient town.

The settlement was founded in the 1st century BCE and served as the main administrative and trading center of the Île-de-France region until the 6th century. In the 12th century, the land was transformed into a Cistercian farm called Ferme d'Ithe, layering medieval remains over the earlier Roman occupation.

The name Diodurum is recorded only in the Itinerary of Antoninus, as the late Latin form Dioduro, built from the Gaulish divo-, "divine", and duron, which moved from "door" to "enclosed market, forum" and finally "walled town". Through Romance sound change the same root produced the modern place-name Jouars. The site lies in the commune of Jouars-Pontchartrain, in the Yvelines.

The site lies on the territory of Jouars-Pontchartrain and Le Tremblay-sur-Mauldre in the Yvelines, near the diverted route of the RN 12, whose roadworks led to the excavations. The remains are spread over open ground, so plan for uneven terrain and outdoor conditions. The Association pour la protection du site archéologique de Diodurum (APSAD) works toward an open-air museum on the site.

Diodurum is known only from a single mention in the Itinerary of Antoninus, where it appears as Dioduro. The Gaulish divo- (divine) combined with duron, which shifted from "gate" to "enclosed market" and then "walled town", produced the modern place-name Jouars. The site was identified in the 1950s but only excavated during work to reroute the RN 12, which led to the founding of the APSAD.