Microbial infections severely damage poultry, which affects their growth and nutritional value. Antibiotics have been added to animal and poultry feed to combat these infections and promote growth, but the extensive use of antibiotics leads to many health disorders in both animals and humans. Thus, researchers have searched for natural alternatives, such as microalgae, that act as antimicrobials in livestock and poultry rations that do not cause any negative impacts on productivity and health. Microalgae are unicellular or multicellular organisms that perform photosynthesis. They are primarily added to avian diets as a source of long-chain n-3 polyunsaturated fatty acids (PUFAs), including docosahexaenoic and eicosapentaenoic acids, and are also a valuable source of protein, microelements, vitamins, and antioxidants. In addition, microalgae act as a coloring agent for egg yolks and skin. Various studies have shown that microalgae, especially Spirulina and Chlorella, can be efficiently used as feed supplements in poultry diets with positive effects on the quality of chicken meat and egg. Microalgae help to increase the level of n-3 PUFAs and carotenoids, which positively influences performance and immune response. This chapter highlights the nutritive value of microalgae, their bioactive molecules, their use in poultry feed, and their biological properties, including their probiotic, prebiotic, antioxidant, antimicrobial, and immunostimulant potentials. The major limitations of microalgae incorporation in poultry nutrition and the future perspectives of microalgae applications are also highlighted.