Catalysts hold promise as tools for chemical protein modification. However, the application of catalysts or catalyst-mediated reactions to proteins has only recently begun to be addressed, mainly in in vitro systems. By radically improving the affinity-guided DMAP (4-dimethylaminopyridine) (AGD) catalysts that we previously reported (Koshi, Y.; Nakata, E.; Miyagawa, M.; Tsukiji, S.; Ogawa, T.; Hamachi, I.J. Am. Chem. Soc. 2008, 130, 245.), here we have developed a new organocatalyst-based approach that allows specific chemical acylation of a receptor protein on the surface of live cells. The catalysts consist of a set of 'multivalent' DMAP groups (the acyl transfer catalyst) fused to a ligand specific to the target protein. It was clearly demonstrated by in vitro experiments that the catalyst multivalency enables remarkable enhancement of protein acylation efficiency in the labeling of three different proteins: congerin II, a Src homology 2 (SH2) domain, and FKBP12. Using a multivalent AGD catalyst and optimized acyl donors containing a chosen probe, we successfully achieved selective chemical labeling of bradykinin B(2) receptor (B(2)R), a G-protein coupled receptor, on the live cell-surface. Furthermore, the present tool allowed us to construct a membrane protein (B(2)R)-based fluorescent biosensor, the fluorescence of which is enhanced (tuned on) in response to the antagonist ligand binding. The biosensor should be applicable to rapid and quantitative screening and assay of potent drug candidates in the cellular context. The design concept of the affinity-guided, multivalent catalysts should facilitate further development of diverse catalyst-based protein modification tools, providing new opportunities for organic chemistry in biological research.