Protein‐conjugates are vital tools in biomedical research, drug discovery and imaging science. For example, functionalised monoclonal antibodies (mAbs) coupled to the desferrioxamine B (DFO) chelate and radiolabelled with 89Zr are used as radiopharmaceuticals for diagnostic positron emission tomography (PET). In this context, protein functionalisation requires the formation of a covalent bond which must be achieved without compromising the biological properties of the mAb. Photochemistry offers new synthetic routes toward protein‐conjugates like 89Zr‐mAbs but to harness the potential of light‐induced conjugation reactions new photoactivatable reagents are required. Here, we introduce two photoactivatable DFO‐derivatives functionalised with an aryl azide (ArN3) for use in light‐activated conjugation and radiosynthesis of 89Zr‐mAbs. Incorporation of a tris‐polyethylene glycol (PEG)3 linker between DFO and the ArN3 group furnished water‐soluble chelates that were used in the one‐pot, photoradiosynthesis of different 89Zr‐radiolabelled protein‐conjugates with radiochemical yields up to 72.9±1.9%. Notably, the DFO‐PEG3 chelates can be readily synthesised in accordance with Good Laboratory Practice (GLP), which will facilitate clinical trials with photoradiolabelled 89Zr‐mAbs.