Macroalgae are macroscopic, multicellular, autotrophic organisms living in marine environments. Because of their widespread availability, accessibility, and efficiency in fixing carbon dioxide through a carbon-concentrating mechanism, they can accumulate nutrients and secondary metabolites with high applicability in industry. Given the growing awareness of finding new sources of bioactive molecules and functional products, many researchers are focusing their efforts on proving the sustainability of using marine macroalgal biomass as a supply for food, cosmetic, pharmaceutical, nutraceutical, and biomedical applications. It is estimated that of the approximately 10,000 species of macroalgae recorded worldwide, only a small fraction of less than 500 species are successfully exploited in industry. Despite the studies aimed at exploring their properties and the raw materials already commercialized, the full scope of their potential has yet to be reached. This chapter, therefore, is dedicated to four major secondary metabolites found in macroalgae: polysaccharides, peptides, terpenes, and phlorotannins.