This protocol details a novel bioconjugation strategy that uses a methanesulfonyl acrylate reagent that is directed to the most reactive lysine on human serum albumin, which enables the construction of chemically defined and stable bioconjugates. The reaction proceeds rapidly and a regioselective modification is achieved using a single molar equivalent of the reagent under biocompatible conditions (37 °C, pH 8.0). Importantly, the bioconjugate retains both the secondary structural content and function of the unmodified protein. During the reaction of the amino group of lysine and the sulfonyl acrylate reagent, methanesulfinic acid is released after the conjugate addition, which then generates an electrophilic acrylate moiety on the protein. This acrylate can be further used for site-specific protein labeling using a synthetic molecule bearing a reactive amine under biocompatible conditions (21 °C, pH 8.0).