The main purpose of this research is to assess the outdoor thermal comfort (OTC) of Belgrade, in order to investigate how distinct bioclimatic conditions influence on the human organism during the summer in urban environments. Universal Thermal Climate Index, which represents the heat stress of the human organism caused by meteorological conditions, was used as an indicator of thermal comfort in the study. The main goal was to monitor changes in the index values, as well as the frequency of various categories of thermal stress during the hottest part of the year (months June, July, and August). For this research, mean daily meteorological data from the Meteorological Observatory Belgrade have been collected for the period from 1999 to 2018. The obtained results show a gradual change in the bioclimatic conditions, which are generally more adverse in July and August. Results also indicate that OTC in Belgrade was considerably reduced, especially in 2007, 2012, 2015, 2017, and 2018.