Heat stress in horses is a physiological condition that occurs when the animal is unable to dissipate body heat efficiently, leading to an imbalance in thermal homeostasis. This condition arises mainly when environmental factors, such as high temperature, high humidity, and intense solar radiation, exceed the body's thermoregulation capacity. In equines, maintaining body temperature depends on mechanisms such as evaporation (sweating and respiration), conduction, convection, and radiation. However, in hot and humid environments—such as those found in tropical regions—the efficiency of these processes is reduced, favoring the accumulation of internal heat. In response, the body activates physiological adaptations, including increased heart and respiratory rate, intense sweating, and elevated body temperature. When prolonged, heat stress compromises the animal's well-being and performance, and can result in dehydration, metabolic disorders, immunosuppression, and even death. In addition, it negatively impacts the productivity, reproduction, and athletic ability of horses. Therefore, early recognition of clinical signs and proper management of the environment and hydration are fundamental to preventing complications and ensuring the health and thermal comfort of horses.