The study analyses spatial and temporal variability of the UTCI index during winter seasons in Poland in the 21st century. The most frequent UTCI thermal stress category was moderate cold stress with a frequency varying from 45.8% in Łeba (Coastal region) to 66.1% in Zielona Góra (Central region). In mountain areas, this class was less frequent (7.312.3%). The second most frequent class was strong cold stress with values from 11.1% in Zakopane (Carpathian region) to 43.4% in Suwałki (North-East region). In mountain areas, very strong cold stress and extreme cold stress occurred frequently (25.0-35.5% and 17.7-52.0%, respectively). Few cases of extreme cold stress were observed on other Polish stations. Thermoneutral zone was not recorded in the mountain area, although on other Polish stations its frequency ranged from 0.1% in Chojnice, Suwałki and Białystok located in northern part of Poland to 4.3% in Zakopane (Carpathian region).