AbstractFormylglycine‐generating enzymes provide a convenient tool for site‐specific protein derivatization. Their ability to oxidize cysteine or serine residues within a defined consensus sequence to Cα‐formylglycine (FGly) allows for the targeted introduction of a unique chemical handle for various bioconjugation reactions. In recent years, oxygen‐dependent FGly‐generating enzyme saw broad use in protein functionalization and the generation of protein conjugates. Yet, the FGly‐generating system AtsB, along with its capability to convert unusual aldehyde tag sequences, remains mostly unused. Herein, the ability of AtsB from Methanosarcina mazei to convert nonclassical aldehyde tags of the SX(A/P)XR‐type and its potential use in bioconjugation chemistry are demonstrated.