The southern boundary between India and the Tibetan Plateau represents a classical case of continental subduction, where the Indian continental lithosphere is subducted northwards beneath the Tibetan Plateau. At the northern boundary, southward subduction of Asian lithosphere beneath the Tibetan Plateau has also been proposed7, but imaging has been hampered by inadequate data quality. Here we analyse the plate tectonic structure of the northern boundary between Tibet and Asia using the S receiver function technique. Our passive source seismic data build on, and extend further northwards, the existing geophysical data from the International Deep Profiling of Tibet and the Himalaya project8, 9, 10. We detect, beneath central and northern Tibet, a relatively thin, but separate, Tibetan lithosphere overriding the flat, southward subducting Asian lithosphere. We suggest that this overriding Tibetan lithosphere helps to accommodate the convergence between India and Asia in central and northern Tibet. We conclude that the Tibetan–Himalayan system is composed of three major parts: the Indian, Asian and Tibetan lithospheres. In the south, the Indian lithosphere underthrusts Tibet. In central and northern Tibet a separate, thin Tibetan lithosphere exists, which is underthrust by the Asian lithosphere from the north.