The position of the K/P boundary in wells in the Western Sirt Basin of Libya is problematic. The biostratigraphic definition of the K/P boundary in the subsurface is based on the extinction of the eponymous taxon of the standard latest Maastrichtian foraminiferal Abathomphalus mayaroensis Zone. We have unambiguously recognised this taxon from the Sirt Basin subsurface sections for the first time. All the latest Maastrichtian foraminiferal assemblages are dominated by planktonic foraminiferids. Some of specimens are pyritised, indicating organic flux to the basin floor, causing oxygen deficiency in the depositional environment. The occurrence of characteristic bathypelagic and some epipelagic species indicates that the environment of deposition is outer shelf–middle continental slope.