Antigen-binding proteins such as nanobodies are extensively used in biomedicine, diagnostics, and as tools for molecular biology. Often such applications require modification of the nanobodies with fluorophores or drugs. Here, we describe a robust method for introduction of aldehyde handles into His-tagged nanobodies and further derivatization of these proteins with hydroxylamine functionalized compounds of interest. The method allows for isolation of nanobodies containing one or more labels.