Herein we report an umpolung strategy for the bioconjugation of selenocysteine in unprotected peptides. This mild and operationally simple approach takes advantage of the electrophilic character of an oxidized selenocysteine (Se?S bond) to react with a nucleophilic arylboronic acid to provide the arylated selenocysteine within hours. This reaction is amenable to a wide range of boronic acids with different biorelevant functional groups and is unique to selenocysteine. Experimental evidence indicates that under oxidative conditions the arylated derivatives are more stable than the corresponding alkylated selenocysteine.Herein we report an umpolung strategy for the bioconjugation of selenocysteine in unprotected peptides. This mild and operationally simple approach takes advantage of the electrophilic character of an oxidized selenocysteine (Se?S bond) to react with a nucleophilic arylboronic acid to provide the arylated selenocysteine within hours. This reaction is amenable to a wide range of boronic acids with different biorelevant functional groups and is unique to selenocysteine. Experimental evidence indicates that under oxidative conditions the arylated derivatives are more stable than the corresponding alkylated selenocysteine.