Selenocysteine (Sec) is an essential amino-acid in living organisms and plays an important role in redox reactions to maintain physiological reducing environment. Because of the strong and unique nucleophilicity of selenol, Sec residues allow chemoselective functionalization, conjugation and ligation [1]. Since biological expression systems are generally not suitable for Sec-containing peptide/protein preparation, solid-phase peptide synthesis (SPPS) using a side-chain protected Sec monomer is the most commonly used method. However, previous Sec protecting groups available for Fmoc-SPPS is limited compared to other proteinogenic amino acids, and their deprotection often needs harsh reaction conditions. Notably, there are very few protecting groups that can be removed under physiological conditions and functional regulation of Sec-containing peptide/protein by using such protecting group has never been reported to date. To address this issue, we began to develop a new Sec protecting group removable under such mild conditions. In this study, we introduced allylic protecting group to the selenol of Sec and demonstrated to be easily removable in a mild aqueous condition by organometal complexes. We have successfully synthesized a new monomer and incorporated it into model peptides via Fmoc-SPPS.