Heat stress (HS) has a significant negative effect on production and affects growth performance and intestinal health in broilers. The current study assessed the effectiveness of adding increased dietary levels of mint leaf powder (MLP) as a feed additive on growth performance, physiological indicators of stress, intestinal morphology and molecular indicators of stress in broilers subjected to HS. A total of 192 male Cobb broiler chicks from day 14 to 42 were randomized in a 2 × 4 factorial design with the treatment being two ambient temperature regimens (thermo-neutral: 25°C vs HS: 35°C) and four levels of MLP (0%, 0.6%, 1.2% and 1.8%). HS decreased final body weight by 13.60% (P < 0.01), feed intake by 15.4% (P < 0.01), and increased breathing frequency by 77.50% (P < 0.001). Supplementation with 1.8% MLP improved body weight (3.4%; P < 0.02), breathing frequency (-25%; P < 0.001), and body temperature (P < 0.044) while experiencing HS groups. HS also decreased ileal villus height area and villus width (P < 0.01); whereas 1.8% MLP supplementation preserved intestinal morphology. Changes at the molecular level showed that HS markedly upregulated of ileal heat shock protein 70 (HSP70) of 3.83-fold in the MLP groups over in the control group (0.96-fold, P < 0.001) and HSP90 (1.73-fold, P < 0.001). MLP supplementation modulated HSP expression; HSF3 decreased (P < 0.012), and had significant degrees of suppression of inflammatory cytokines, specifically tumour necrosis factor alpha (TNF-α) expression by 68% under HS (P < 0.001). Results concluded that using 1.8% MLP as a continuous phytogenic additive is effective in mitigating HS by mechanisms involving multi-level amelioration of HS, improved thermoregulation, intact intestine morphology, regulated molecular responses to stress, and powerful anti-inflammatory properties.