Thirty percent of a cell's volume is filled with macromolecules, and protein chemistry in a crowded environment is predicted to differ from that in dilute solution. We quantified the effect of crowding by globular proteins on the equilibrium thermodynamic stability of a small globular protein. Theory has long predicted that crowding should stabilize proteins, and experiments using synthetic polymers as crowders show such stabilizing effects. We find that protein crowders can be mildly destabilizing. The destabilization arises from a competition between stabilizing excluded-volume effects and destabilizing nonspecific interactions, including electrostatic interactions. This competition results in tunable stability, which could impact our understanding of the spatial and temporal roles of proteins in living systems.