The complexation of partially ethyl-quaternized poly(4-vinyl pyridine) (QVP) and poly(methacrylic acid) (PMAA) induces a dramatic change in solution viscoelasticity. In this work, we investigated a model system consisting of QVP (with 2% charge), PMAA, and dimethyl sulfoxide (DMSO)-water mixed solvent (85 wt % DMSO-15 wt % water), which exhibits a remarkable thermothickening behavior, characterized by an increase of viscosity by 3 orders of magnitude when the temperature is increased by 20 'C. At low temperatures, this system behaves as a low-viscosity milky colloidal suspension that remains stable for periods of at least 1 year. At higher temperatures, the colloids swell, increasing their effective volume fraction and giving rise to the observed viscosity increase. The thermothickening/thermothinning transition temperature could be tuned by varying the stoichiometry of the mixed polymers. We utilized rheometry, UV vis spectroscopy, and dynamic light scattering to shed light on the mechanism of this phenomenon. This simple approach for achieving tunable thermothickening capability provides a new platform for designing thermoresponsive solutions from simple polymer mixtures.