Updated project metadata. Optimum protein function and biochemical activity critically depends on water availability, because solvent thermodynamics drive protein folding and macromolecular interactions. Reciprocally, macromolecules restrict the movement of “structured” water molecules within their hydration layers, reducing the available “free” bulk solvent and therefore the total thermodynamic potential energy of water, or water potential. Within concentrated macromolecular solutions like the cytosol, we found modest changes in temperature greatly impact the water potential, and are counteracted by opposing changes in osmotic strength. Remarkably, this duality of temperature and osmotic strength allows simple manipulations of solvent thermodynamics to prevent cell death upon extreme cold or heat shock. Physiologically, cells must sustain their activity against fluctuating temperature, pressure and osmotic strength that impact water availability within seconds. Yet, established mechanisms of water homeostasis act over much slower timescales, so we postulated the existence of a rapid compensatory response. We find this function is performed by water potential-driven changes in macromolecular assembly, particularly biomolecular condensation of intrinsically-disordered proteins. Formation or dissolution of biomolecular condensates liberates and captures free water, respectively, quickly counteracting thermal or osmotic perturbations of water potential, which in consequence is robustly buffered in the cytoplasm. Our results indicate biomolecular condensation constitutes an intrinsic biophysical feedback response that rapidly compensates for intracellular osmotic and thermal fluctuations. We suggest preserving water availability within the concentrated cytosol is an overlooked evolutionary driver of protein (dis)order and function.