Trapa L. (Lythraceae J. ST.-HIL) is a genus of annual free-floating plant, widely distributed in the Northern Hemisphere. The Qaidam Basin is regarded as an ideal region to understand the Cenozoic palaeoclimatic and palaeobiological evolution of the northern Tibetan Plateau. A few records of Trapa have been previously mentioned from the Cenozoic deposits of the Qaidam Basin, northern Tibetan Plateau. Here, we describe some fossil fruits of Trapa from the late Miocene Shangyoushashan Formation of southeastern Qaidam Basin (Tuosu Lake area), which were identified as Trapa qaidamensis sp. nov. based on their morphology. Extant species of Trapa usually live in the environments with mean temperature of warmest quarter (MTWQ) ranging from 14.9 degrees C to 30.9 degrees C, as well as mean temperature of warmest month (MTWM) ranging from 19.6 degrees C to 37.3 degrees C in the living areas of Trapa. Compared with MTWQ 16.5 degrees C and MTWM 17.5 degrees C in current fossil locality, the present fossil site was warmer during the Miocene than in the present day. [GRAPHICS] .