Belonging to the Staphylinidae, the largest animal family known, recent Omaliinae are a diverse and widespread group of rove beetles. There are omaliine representatives known since Early—Middle Jurassic compressions, but members of the tribe Omaliini have been known only from the Cenozoic. Duocalcar geminum Peris and Thayer gen. et sp. nov. is described as the oldest definitive fossil of the tribe Omaliini worldwide, originating from opaque mid-Cretaceous (latest Albian) amber of Charentes, south-western France. The discovery and description were made possible with the use of the propagation phase-contrast X-ray synchrotron imaging technique, which allows the detailed study of otherwise invisible specimens in opaque amber.