Amino groups are common in both natural and synthetic compounds and offer a very attractive class of endogenous handles for bioconjugation. However, the ability to differentiate two types of amino groups and join them with high hetero‐selectivity and efficiency in a complex setting remains elusive. Herein, we report a new method for bioconjugation via one‐pot chemoselective clamping of two different amine nucleophiles using a simple ortho‐phthalaldehyde (OPA) reagent. Various α‐amino acids, aryl amines, and secondary amines can be crosslinked to the ϵ‐amino side chain of lysine on peptides or proteins with high efficiency and hetero‐selectivity. This method offers a simple and powerful means to crosslink small molecule drugs, imaging probes, peptides, proteins, carbohydrates, and even virus particles without any pre‐functionalization. Simple ortho‐phthalaldehyde (OPA) reagent enables a facile and broadly applicable bioconjugation strategy via hetero‐selective clamping of two different endogenous amino handles such as ϵ‐NH2 of lysine and α‐NH2 of α‐amino acids without any pre‐functionalization.