In this study, the Universal Thermal Climate Index (UTCI) gridded data from ECMWF (European Centre for Medium-Range Weather Forecasts) database was applied to assess the spatiotemproal changes in thermal comfort in Iran for 1990–2021. This index's monthly changes and trends have been determined, using the Mann-Kendall nonparametric test. Moran's I was used to compute the index's spatial autocorrelation, and the Getis-Ord Gi* statistic was used to calculate the index's hot and cold spots. The findings indicated that the majority of the country experiences thermal comfort condition from June to October. Furthermore, no location of Iran has recorded very hot or very strong heat stress. Moderate and strong heat stress has only been recorded in a few areas along the southern coasts from May to November. Most of the country experiences strong, moderate, or slight cold stress from January to May, as well as November and December. According to the Moran I autocorrelation index, the spatial pattern of UTCI in Iran is cluster throughout the year. Significant hot spots have also formed throughout the country's southern coasts, with limited spatiotemproal variability. Significant cold spots have also formed, with more temporal and spatial variability, along the Alborz and Zagros mountain ranges.