The bioconjugation of proteins to small molecules serves as an invaluable tool for probing biological mechanisms and creating biomaterials. The most powerful bioconjugation stratagems are those which have rapid kinetics, are site-selective, can be conducted in aqueous solvent under mild conditions and do not require the use of additional potentially toxic catalysts. Herein we present spontaneous coupling via aldol ligation of proteins (SCALP) a catalyst-free “green” site-selective protein ligation between α-oxo aldehyde functionalised proteins and enolisable aldehyde probes at neutral pH, and demonstrate the utility of this system in the targeting of prostate cancer cells with functionalised nanobodies.