Microalgae are present in aquatic surroundings. These unicellular and photosynthetic microorganisms are attracting the attention of researchers and industries in different areas, such as pharmaceutical and nutraceutical, due to their content of antioxidant and bioactive molecules, such as polyunsaturated fatty acids (PUFAs), carotenoids (pigments), polysaccharides, vitamins, and phenolic compounds. These antioxidants have shown the ability to reduce or inhibit oxidative stress, which has been strongly associated with the appearance of several chronic diseases, such as diabetes, autoimmune disorders, cardiovascular ailments, and cancer, among several others. In addition to Chlorella sp., Dunaliella sp., and Spirulina sp., which are the most studied microalgae species, other species are worth exploring as sources of antioxidant and bioactive substances. This chapter aims to identify microalgae species that can be used as sources of bioactive compounds with antioxidant and nutraceutical properties.