As the utility of proteins in medical diagnosis and therapy becomes more fully realized, interest in characterizing proteins’ stability continues to increase. This paper reports the merits of rheology as an approach to characterizing the stability of a model protein, immunoglobulin (IgG). A controlled-strain rotational rheometer equipped with parallel plates geometry was used to measure changes in the viscosity of an IgG buffer solution during a programmed temperature ramp. The results demonstrated that protein stability characterization is reproducible using the melting temperature (or unfolding temperature) of IgG (Tm = 73.6 ± 0.4 °C). We have also identified an irreversible minor transition occurring at a temperature of 40–60 °C, which we believe to be the partial unfolding of some protein structures. Finally, we have determined that adding sugar, an established protein stabilizer, to the IgG buffer solution significantly increases the protein's melting temperature. Our results show the power and simplicity of rheology as analytical tool for characterizing protein stability through the measurement of viscosity changes during thermal denaturation.