This study investigated the impact of heat stress (HS) on growth performance, carcass traits, telomere length (TL), and gene expression profiles in three chicken breeds with varying growth rates: slow-growing (SAGA), medium-growing (Sasso), and fast-growing (Cobb 500). Three hundred 14-day-old male chicks were exposed to either control (25°C) or HS (34°C for 6 hours/day) conditions for four weeks in a controlled environment. Weekly growth metrics, TL at two and four weeks, Heat Shock Protein 70 (HSP70) and Insulin-Like Growth Factor-1 (IGF-1) expression in muscle and liver at two and four weeks of HS exposure, and carcass/organ yields at four weeks were analyzed. Cobb 500 chickens exhibited significant growth reductions under HS, while SAGA showed resilience. Notably, SAGA chickens exhibited a significant increase in intestinal organ mass under HS, which may indicate an adaptive response to thermal stress. HS exposure significantly shortened TL across all breeds, suggesting its utility as a universal biomarker for HS in chickens. All breeds upregulated HSP70 expression, with the Cobb 500 showing the most prominent increase. Similarly, IGF-1 was expressed (upregulated), particularly in 500 broilers at both time-points, highlighting breed-specific differences in growth performance. These results demonstrate breed-specific physiological adaptations to HS. TL and stress-related gene expression are crucial indicators of heat susceptibility and adaptation. The study provides insights into developing breed-specific management and breeding strategies to enhance poultry resilience to increasing global temperatures.