<p>Thiols are used in many click reactions, and are also excellent platforms for biomolecular click or bioconjugation reactions. The direct cross-coupling of two thiols is an attractive biomimetic concept for click chemistry, but leads to statistical mixtures of homo- and heterodimers. Here, we introduce a novel class of thiol-click reagents, bromo-ynones, where the kinetic differentiation between the first and second thiol addition onto these reagents facilitates a stepwise one-pot cross-clicking of two distinct thiols in aqueous media, without the need for intermediate isolation or purification. The two thiols are linked through a single carbon atom, mimicking a disulfide bridge. We demonstrate the use of bromo-ynones in the synthesis of various cross-coupled thiols, including small molecule drugs, fluorophores, carbohydrates, peptides and proteins, including an example of a protein-protein heterodimer. The resulting adducts are robust under physiological conditions and by judicious choice of the bromo-ynone reagent, the adducts can be stable even in the presence of excess free thiols.</p>