Selenopeptides can be transamidated by cysteinyl peptides in water using mild conditions (pH 5.5, 37 °C) in the presence of an arylthiol catalyst. Similar conditions also catalyze the metathesis of selenopeptides. The usefulness of the selenophosphine derived from TCEP (TCEP═Se) for inhibiting the TCEP-induced deselenization of selenocysteine residue is also reported.