Due to climate change crisis, the risk of occupational heat stress for agricultural workers has recently increased. The temporal and spatial biometeorological conditions in different climatic regions of Slovenia during summer were analyzed using the Universal Thermal Climate Index (UTCI), and additionally the water loss index (SW) and the accepted level of physical activity (MHR). Term values of air temperature, relative air humidity, wind speed at 10 m and cloud cover at 14:00 CEST were used as input for the BioKlima 2.6 software package and were retrieved from the Slovenian Environment Agency for the summer months in the period 2000–2021. The rise in the average UTCI values was shown to be positive and statistically significant for summer (0.7 °C/decade) as well as for all three months, the highest being for June (0.9 °C/decade). The percentage of summer days during 2000–2021 that were under strong or very strong heat stress varied widely by location, ranging from one-third to more than one-half. Average monthly UTCI values at 14:00 CEST were the highest in July, reaching 30 °C in a submediterranean climate, Črnomelj is the only station with this average higher than 32 °C. Daily highest UTCI value was 47 °C (Črnomelj). It was shown that conditions in the middle of a hot summer day are not suitable for moderate or severe agricultural workloads.