The end-Permian mass extinction has been associated with severe global warming. Main stage volcanism of the Siberian Traps occurred at or near the extinction interval and has been proposed as a likely greenhouse catalyst. In this study, a high-resolution δ¹⁸O record is established using diagenetically resistant apatite of conodonts and low-Mg calcite of brachiopods from stratigraphically well-constrained Permian-Triassic (P-Tr) boundary successions in northwestern Iran. A new evaluation is made for previously published conodont δ¹⁸O values from South China and revised palaeotemperatures are presented together with new data from Wuchiapingian to Griesbachian sections in Iran. δ¹⁸O data from P-Tr sections in Iran document tropical sea surface temperatures (SST) of 27-33°C during the Changhsingian with a negative shift in δ¹⁸O starting at the extinction horizon, translating into a warming of SSTs to over 35°C. The results are consistent with re-calculated SSTs of the South Chinese sections. Warming was associated with an enhanced hydrological cycle involving increased tropical precipitation and monsoonal activity in the Tethys Sea. Global warming, intensification of the hydrological cycle and associated processes, vertical water column stratification, eutrophication and subsequent local anoxia may all have facilitated an extinction event.