3-(Methylseleno)-L-alanine, also known as Se-methylselenocysteine, is a naturally occurring organoselenium compound and a major dietary form of selenium. It is a non-proteinogenic amino acid derivative where a methylseleno group (-SeCH3) replaces the sulfur atom in L-cysteine. This compound is a key precursor for the synthesis of the critical antioxidant enzyme glutathione peroxidase and other selenoproteins. Its primary significance lies in nutritional biochemistry and preventive medicine. It is a highly bioavailable and less toxic source of selenium compared to inorganic forms like selenite. Research indicates it possesses potent chemopreventive properties against various cancers, acting through mechanisms such as induction of apoptosis, cell cycle arrest, and inhibition of angiogenesis. It is also studied for its roles in antioxidant defense, immune modulation, and thyroid hormone metabolism. Due to its biological activity and safety profile, it is utilized as a functional ingredient in dietary supplements and nutraceuticals. In pharmaceutical formulations, it is explored as a novel excipient or active component in specialized delivery systems aimed at cancer prevention or selenium supplementation therapies.