The field of protein bioconjugation has developed rapidly over the past three decades and a plethora of techniques now exist for modifying the side-chains of canonical amino acid residues such as lysine , tyrosine, tryptophan, methionine, histidine, and cysteine. However, the targeting of canonical amino acids in proteins is seldom site-specific, as multiple copies of the same amino acid may be solventexposed. To overcome these limitations, methods have been developed that enable site-specific installation of bioorthogonal functionalities, such as alkynes, azides, tetrazines and aldehydes, into proteins. These bioorthogonal motifs can subsequently be conjugated to rapidly and selectively. C-C bond formation represents a gold-standard in the formation of stable bioconjugates. Aldol condensations are one of the most fundamental tools for C-C bond formation in organic synthesis, and accordingly conjugation to protein aldehyde functionalities via aldol/enolate-type chemistries represents a popular method via which C-C bond formation between small molecule probes and proteins can be achieved. Existing examples of such bioconjugation chemistries include Knoevenagel condensation, aldol condensation using 2,4-thiazolidinediones as nucleophilic donors and organocatalyst-mediated protein aldol ligation. Herein we introduce spontaneous coupling via aldol ligation of proteins (SCALP), a facile and gentle site-selective C-C bond forming bioconjugation method between reactive α-oxo aldehyde functionalities on proteins and enolisable aldehyde probes (Figure ). Whereas many aldol-ligations require the usage of organocatalysts based on proline, catalysing aldol condensation via enamine formation, SCALP adds to the repertoire of catalyst-free aldol C-C bond ligations, and can be performed in aqueous solution at neutral pH. SCALP is simple, clean, cheap, and atom-economical, and its site-selective, making it ideal for use in the preparation of small moleculeprotein bioconjugate therapeutics, such as antibody or nanobody drug conjugates. Nanobodies are promising scaffolds for antibody-based therapies and can potentially reduce the costs and bottlenecks arising from monoclonal antibody production by obviating the need for complex mammalian expression systems. JVZ007 is a nanobody capable of high affinity binding to the extracellular domain of surface prostate-specific membrane antigen (PSMA), which is overexpressed on prostate cancer cells. We showcase the SCALP platform by modifying the JVZ007 nanobody site-selectively to yield a functional agent capable of labelling prostate cancer cells. We also demonstrate that bioconjugates labelled using SCALP can be subjected to further downstream bioconjugation reactions, such as azide-alkyne Huisgen cycloadditions.