The assessment of thermal stress stimuli in Poland was based on the Universal Thermal Climate Index (UTCI). Daily values of UTCI at 24 stations selected to represent the whole area of Poland at 12:00 UTC were calculated over the period 1951-2018. Various UTCI characteristics, i.e., spatial averages of daily, monthly and annual values, daily spatial contrasts (i.e., the difference between daily highest and lowest UTCI values) were defined in each of 8 bioclimatic regions of Poland. Ten-year trends for UTCI and spatial contrast values were counted and their statistical significance was analysed. A statistically significant increase in UTCI values was found in all the regions of Poland. However, contrasts in thermal stress both for Poland as a whole and in most of the regions decreased significantly, especially in the north-east of Poland, the country’s coldest region. This indicates a reduction in the stimuli of thermal environment in Poland and in most of the individual regions.