Heat stress (HS) endangers the quality and yield of poultry products and compromises the sustainability of poultry production. Birds are particularly sensitive to HS due to the absence of sweat glands, limited respiratory and cardiovascular systems development, and increased metabolic rates. The gastrointestinal tract (GIT) is one of the main organs affected by HS via numerous pathways, including organ ischemia and hypoxia, resulting from superficial vasodilation and excessive panting. Additionally, HS damages epithelial cells, negatively affects the GIT function in nutrient resorption and digestion, causes immunosuppression, destroys microbial balance, and damages the intestinal barrier and microstructure. Thus, HS disrupts intestinal homeostasis and induces gut permeability (leaky gut), causing systemic inflammation and infection. Heat stress induces the disruption of tight junctions and oxidative stress by the overproduction of reactive oxygen species (ROS), damaging intestinal tissues. Heat stress decreases the creation of digestive enzymes owing to high levels of ROS that augment lipid peroxidation. Increasing the production of heat shock proteins can be considered a sign of intestinal tissue injury. This review aims to deliver a profound view of the effect of thermal stress on the integrity and function of the gastrointestinal tract.