This study investigated the effects of supplementation with a liquid organic acid blend (OAB) on the growth performance, intestinal morphology, blood biochemistry and susceptibility to ascites in Cobb 500 broilers exposed to heat stress. A completely randomised design was applied with three treatments: no OAB supplementation (control, CONT), OAB supplementation (0.2%) during the entire period (FP) (days 1–42, OAB/FP) and OAB supplementation (0.2%) during the late period only (LP) (days 21–42, OAB/LP). To induce heat stress, room temperature was maintained at 28–30°C. At 42 days of age, the final body weight of broilers in OAB/FP and OAB/LP groups was higher (2632.6 and 2569.4 g, respectively), than in controls (2383.2 g, respectively, P < 0.01). Improved feed conversion ratio values at 42 days of age were also observed in the OAB/FP (1.71) and OAB/LP (1.75) groups compared to the CONT group (1.92, P < 0.01). OAB supplementation stimulated villus growth in the small intestine (P < 0.01). Mortality associated with ascites was highest in the CONT group (11.3%), followed by the OAB/FP (3.8%) and OAB/LP (1.3%) groups (P < 0.05). The supplementation with OAB increased immunoglobulin concentrations and decreased the heterophil-to-lymphocyte (H/L) ratio, a stress indicator (P < 0.05). In conclusion, organic acid blend supplementation improved productive performance, villus growth, and immune status, and reduced both ascites-related mortality and stress level during broiler exposure to high temperatures. Supplementation throughout the entire rearing period (days 1–42) appeared to be more effective as protection against heat stress.