Marine seaweeds produce protective and defensive compounds, for example, carbohydrates and phenolic compounds that are a source of bioinspiration for molecules with cosmetic and human health applications. This chapter provides a general description of seaweeds, together with a brief overview of the adaptive and chemical defenses developed by these organisms. Among the great diversity of metabolites produced by seaweeds, we focus on specific carbohydrates (floridoside and mannitol) and phenolic compounds (mycosporine-like amino acids and phlorotannins) produced by macroalgae. Particular attention is paid to their highly variable chemical structure and methods for their extraction and purification. Their mechanisms of action are discussed, focusing on osmoregulation, desiccation, and photoprotection, which affect seaweed ecophysiology. Finally, we review their utility for cosmetology as moisturizers, sunscreens, and antiaging agents, for human health as wound dressings, and for skin regeneration. We point out the challenges faced when using these natural compounds in the cosmetic and human health sectors and the problems linked to the exploitation of this natural resource.