Sortase‐mediated hydrazinolysis of proteins with hydrazine or its derivatives was developed for the production of recombinant protein hydrazides. This process provides an alternative approach for protein semisynthesis through the use of recombinant protein hydrazides as thioester surrogates. It also provides an alternative method for C‐terminal modification of proteins with functional units as well as for the preparation of C‐to‐C fusion proteins. Takes all sort(ase)s: Sortase A mediated hydrazinolysis reactions of proteins with hydrazine or its derivatives were developed as an efficient process for the production of recombinant protein hydrazides. This reaction provides an alternative approach for the semichemical synthesis as well as C‐terminal modification of proteins.