Medicinal plants with active ingredients have shown great potential as natural and sustainable additives in livestock and poultry diets as growth promoters, performance, feed conversion ratio, digestibility of nutrient enhancers, and antioxidants and immune system modulators. Among active ingredients, phytosterols, which are plant-based bio-factors that may be found in seeds, fruits, grains, vegetables and legumes, are thought to be involved in the aforementioned activities but are also widely known in human medicine due to their efficacy in treating diabetes, coronary heart disease, and tumors. Nevertheless, phytosterols can also promote carcinogens production, angiogenesis inhibition, metastasis, infiltration, and cancer cells proliferation. This review focuses on the deepening of the biological role and health benefits of phytosterols and their new potential application in poultry and livestock nutrition.