Human beings are adversely affected by climate extremes, pertinent to an increase in frequency and intensity of warm temperatures, eventually inducing warming on a global and regional scale. In a tropical nation like India, high summer temperature and increased moisture with the arrival of the southwest monsoon (hereafter referred to as monsoon) aggravate the sultriness of the ambient environment. Irrespective of global climate change, cities alter their climate due to urban materials' impervious surfaces and thermal properties, which upsurge moisture and temperature in urban settings. Thus, urban dwellers are peculiarly vulnerable to heat stress health hazards. Heat stress indices allow quantitative assessment of thermal stress to determine the safe limits of thermal exposure. In the present study, statistical trends in Heat Index were evaluated to analyze heat stress over 41 urban stations of southern peninsular India over the summer and monsoon season from 1969 to 2015. Results indicated that almost all stations registered a significant increase at 95% confidence level in heat stress except for an insignificant decrease at a few stations. Changepoint detection depicted an increase in heat stress initiated in the late 1990s and early years of the decade 2000 at most urban stations. Hierarchical cluster analysis partitioned data into seven spatial units. Accordingly, the highest magnitude of increase was observed over cities located in the northeastern part of the study area and the southern tip of peninsular India. The study demands attention to perilous health risks related to India's increasing heat stress casualties and the need for an indigenous thermal stress alerts system.