The upper-mantle structure across the Zagros collision zone, in southwest Iran, is investigated using a non-linear weighted damped least-squares teleseismic tomography approach. The resolution of the structures/transitions in the upper mantle is enhanced significantly by correcting the teleseismic relative arrival time residuals for an a priori crustal velocity model and then performing the inversion with fixed crustal blocks. To investigate whether or not the lithospheric blocks and major transitions in the resulting model are required by the data or are artefacts of the inversion, the data were inverted using two different inverse methods (singular value decomposition and a quadratic programming method). New high-quality seismic velocity models show apparent correlation between surface geological features and seismic velocity structures at lithospheric depth across the Zagros collision zone. The image shows a sharp lithospheric boundary at the Main Zagros Thrust between 100 km and 250 km depth with P-wave velocity about 3 per cent faster within the Arabian Shield to the south. A step-like increase in lithospheric thickness across the Zagros collision zone is assumed to separate two different mantle structures namely the Arabian (to the south) and the Eurasian (to the north) domains. The most striking feature resolved is a north-dipping slab-like positive velocity anomaly.