The unprecedented rate of global warming requires more immaculate strategies to fight the heat stress and its detrimental effects on poultry sector. Nutritional strategies, particularly herbal extracts, play a crucial importance in this scenario. Therefore, this study was conducted to evaluate the effects of a novel selected mixture (hereafter refers to as Stress-Bio-Max,
SBM
) comprising St John's wort, lavender,
Melissa officinalis
extracts, betaine, and Vit C on heat-stressed broilers. A total of 300 male broiler chicks were allocated to 4 treatments: T1, Control (no SBM in drinking water); T2, 0.25 mL/L of SBM in drinking water; T3, 0.5 and T4, 1 mL/L of SBM, which applied since d 32. The birds underwent heat stress (40 ± 1) daily from d 35 to 42 of the experiment (2 pm–8 pm). The results show the superior weight gain of T2 and T3 from d 7 to 42 compared to other groups, 72.5 and 72.7 g/d, respectively (
P
< 0.05). Corticosterone level was higher on d 35 in T1 (4.674 ng/mL,
P
< 0.05) and lower in T2 and T3 (2.64 and 2.952 ng/mL, respectively,
P
< 0.05); T2 and T3 also caused the lowest concentrations of corticosterone on d 40 (5.198 and 6.458 ng/mL, respectively,
P
< 0.05). The superior triiodothyronine levels belonged to T2 (0.935 ng/mL,
P
< 0.05) on d 35 and T2 and T3 on d 40 (0.699 and 0.582 ng/mL, respectively,
P
< 0.05). T2, T3, and T4 caused a rise in glutathione peroxidase (
GPX
) and superoxide dismutase (
SOD
) activities (
P
< 0.05). A significant higher amount of total antioxidant capacity (
TAC
) belonged to the groups treating with SBM (
P
< 0.05). The SBM in T2 and T3 resulted in the lowest levels of malondialdehyde (
MDA
), aspartate transaminase (
AST
), alanine aminotransferase (
ALT
), and alkaline phosphatase (
ALP
). Results from this study indicate that SBM may alleviate the negative impacts associated with heat stress in broiler chickens.
