The climate and thermal comfort of а destination greatly influence the tourism industry. Therefore, this study was focused on researching thermal comfort changes and their impacts on visitors in four highly visited coastal destinations along the eastern Adriatic coast (Pula, Zadar, Split, and Dubrovnik) from 1996 to 2020, using the modified physiologically equivalent temperature index (mPET). The specific objective was to assess how the thermal comfort conditions are distributed spatially and temporally and how they are suitable for beach and sightseeing tourism. Results showed that monthly mean mPET values have increased, except in May. In the summer season, tourists were often exposed to uncomfortable heat stress, especially in the middle of the day. Strong and extreme heat stress frequency significantly increased in all sites except in Pula, particularly in July and August. Prevailing neutral and slightly warm/cold conditions were concentrated in two periods, between April and June and in September and October. The maximum occurrence of optimal climatic conditions for enjoying the beach was in the summer, with a decreasing tendency from May to August. The occurrence of favorable conditions for sightseeing significantly increased in April and November while it decreased from July to September. Although the eastern Adriatic coast is primarily a summer tourist area, a relatively small number of tourists take advantage of the period with optimal thermal comfort. Despite the fact that the number of tourists in the shoulder seasons has increased significantly in the past 25 years, the increasing favorable thermal comfort in the changing climate conditions will make these seasons even more appealing in the future, especially for sightseeing activities. New strategies for adapting to a changing climate are therefore needed.