Osmolytes (small molecules that help in circumventing stresses) are known to promote protein folding and prevent aggregation in the case of globular proteins. However, the effect of such osmolytes on the structure and function of intrinsically disordered proteins (IDPs) has not been clearly understood. Here we have investigated the effect of methylamine osmolytes on α-casein (an IDP present in mammalian milk) and discovered that TMAO (Trimethylamine-N-oxide) but not other methylamines renders α-casein functionless. We observed that the loss of chaperone activity of α-casein in presence of TMAO was due to the induction of an unstable aggregation-prone intermediate. The results indicate that different osmolytes may have different structural and functional consequences on IDPs, and therefore might have clinical implications for a large number of human diseases (e.g., amyloidosis, cancer, diabetes, and neurodegeneration) where IDPs are involved.