Microalgae are aquatic resources containing several value-added products which have been and will be widely used in various industries. They have the high potential to be explored in cosmetics and cosmeceuticals, as active ingredients in the form of isolated molecules or extracts. The aim of this review is to exhibit the potential of microalgae and especially diatoms for their cosmetic application regarding their phytochemistry and pharmacological activity. Comprehensive study of previously published literature demonstrated the presence of several antioxidant, antimicrobial or anti-inflammatory molecules in microalgae and their efficiency on human cells, particularly in anti-aging and anti-UV products. This review summarizes and discusses the information on extraction process, the chemical composition and biological activity of natural microalgae biomass. Finally, the review attempts to reveal more global view on how to valorize natural microalgal biomass, including diatoms, in cosmetics to prevent a destruction of wild resources.