VCs show a wide range of pharmacological properties; the most important application in medicine is for the treatment of cancer and diabetes. The VCs therapeutic action may be associated with their binding to proteins. For this reason, the characterization of VCs/protein interactions is important. Among the most promising VCs, dioxidovanadium(V) complex with the aroylhydrazone furan-2-carboxylic acid 3-ethoxy-2-hydroxybenzylidene)hydrazide (GSW-4) deserves to be mentioned. GSW-4 is cytotoxic for several cancer cell lines, including HeLa. The interaction of GSW-4 with the model protein hen egg white lysozyme (HEWL) was studied by X-ray crystallography. X-ray diffraction data, collected under two different experimental conditions, reveal that GSW-4 and V-containing fragments derived from this molecule bind the protein through non-covalent interactions with the side chains of Arg5, Cys6, Glu7 and Lys33. On the protein surface, a GSW-4 molecule forms a supramolecular association with another GSW-4 unit through a Na⁺ ion. The supramolecular assembly is stabilized by stacking interactions. The reactivity of GSW-4 with HEWL could help in understanding of transport and mechanisms of action of this molecule, promoting the development of new compounds as therapeutic agents.