The supramolecular formation of a PEGylated bovine serum albumin (BSA) protein-polymer bio-conjugate in water has been demonstrated through a selective host–guest interaction with the macrocycle cucurbit[8]uril (CB[8]). Both BSA and poly(ethylene glycol) were functionalised with either an electron-deficient first guest viologen or an electron-rich second guest naphthalene for the formation of the CB[8] ternary complex. With the help of spectroscopic (NMR, DOSY-NMR, DLS, UV/vis, fluorescence) and calorimetric (ITC) techniques, it was shown that a strong and specific binding interaction took place between the complementary labeled polymer and protein only in the presence of the macrocyclic host CB[8]. Moreover, we demonstrated that controlled formation of a supramolecular protein-protein complex was also possible through the use of CB[8] ternary formation.