This study projects absolute temperature extremes using the Combined Thermal Hazard Index (CTHI). An evaluation of the CMIP6 multi-model ensemble (CMIP6-MME), comprising 14 bias-corrected models, reveals pronounced spatial heterogeneity in its ability to reproduce absolute temperature extremes during the historical period (1990–2014). Based on the refined Index of Agreement, the model performance for absolute maximum temperature (TXx) exceeds that for absolute minimum temperature (TNn). Although model performance degrades across complex topographic zones such as the Zagros Mountains, the model shows strong agreement with observational data in the interior regions. The projections indicate widespread warming across the region, with no negative temperature anomalies. In the near term, climate system inertia yields a convergent thermodynamic response; however, significant divergence among emission scenarios emerges toward the end of the century. Specifically, the spatially averaged anomalies of TNn and TXx stabilize between 1.8 °C and 2.0 °C under the SSP1-2.6 scenario, but reach 4.4 °C to 5.7 °C under SSP3-7.0 and exceed 8.0 °C under SSP5-8.5. The locus of these extreme changes—where anomalies exceed 8.0 °C—is concentrated over the Zagros, Alborz, and northwestern highlands, driven directly by the elevation-dependent warming (EDW) mechanism and the snow-albedo feedback. By the end of the century, CTHI values stabilize at low levels (maximum 0.25) under SSP1-2.6, but exceed 0.8 across large parts of the country under SSP5-8.5. This shift in the thermal regime, coupled with reduced snow-cover persistence and intensified actual evapotranspiration, will pose irreversible challenges to the water security of these basins.