The ability of Bacillus subtilis transglutaminase (bTG) to functionalize BSA has been investigated using peptide mapping experiments. Interestingly, the conjugation was not detected on a glutamine but on an asparagine residue. A sequence determination study was further performed, and a sequence of 10 amino acids for site-specific conjugation was identified. A monobody showing no native reactivity with the bTG enzyme was produced with the identified peptide sequences and successfully conjugated to various types of substrates in very high yields (>90%) with a 1/1/1.5 ratio of protein/amine/enzyme. Direct conjugation to the amino linker of a small interfering RNA (siRNA) was achieved in good yield, and no impact on the siRNA activity was observed following the conjugation. The identified sequences were further engineered in VHH and IgG scaffolds, and successful conjugation could also be observed with both small molecules and siRNA, confirming the potential of bTG for site-specific enzymatic bioconjugation.