Chemoenzymatic modification of proteins is an attractive option to create highly specific conjugates for therapeutics, diagnostics, or materials under gentle biological conditions. However, these methods often suffer from expensive specialized substrates, bulky fusion tags, low yields, and extra purification steps to achieve the desired conjugate. Staphylococcus aureus sortase A and its engineered variants are used to attach oligoglycine derivatives to the C-terminus of proteins expressed with a minimal LPXTG tag. This strategy has been used extensively for bioconjugation in vitro and for protein− protein conjugation in living cells. Here we show that an enzyme variant recently engineered for higher activity on oligoglycine has promiscuous activity that allows proteins to be tagged using a diverse array of small, commercially available amines, including several bioorthogonal func- tional groups. This technique can also be carried out in living Escherichia coli, enabling simple, inexpensive production of chemically functionalized proteins with no additional purification steps.