Hydrocarbon seeps represent some of the most extreme marine habitats but are also home to rich communities developed around chemosynthesis rather than photosynthesis. Here we describe the outcrop of Sahune (Drôme department, south-eastern France), that illustrates a new seeping site during the Late Jurassic (middle Oxfordian), as formally demonstrated by geochemical proxies. We report the associated fauna composed of foraminifers, radiolarians, crinoids, echinoids and ostracods that all point to seepage at bathyal depth. The foraminifer assemblage and the occurrence of the irregular echinoid Tithonia oxfordiana together point to a middle Oxfordian age. We provide an in-depth analysis of the ostracod community, which is the oldest so far reported in such environments. The new species Procytherura praecoquum may be cognate to the seepage site and could illustrate the oldest known example of pore clusters, sometimes proposed as representing ectosymbiosis. The Sahune assemblage demonstrates that cold seep ostracod communities were already a mixture of taxa from platform and deep-sea oligotrophic environments. The post-Jurassic diversification of ostracods at cold seeps was related to colonization events and diversification of families that have been inhabitants of such ecosystems at least since the Oxfordian. The Sahune record changes our current conception of the deep-sea colonization by the ostracods Tethysia and Procytherura that occurred earlier than traditionally considered.