Macromolecular coacervate complexation via spontaneous liquid-liquid separation has become a facile supramolecular assembly approach to produce new functional nanomaterials in aqueous media. Distinct from conventional polyelectrolyte coacervates, the coacervate complexes formed between net neutral zwitterionic polymers and inorganic anionic nanometer-scaled polyoxometalates (POMs) in salted solutions exhibit unique salt hardening and critical gel behaviors with significantly enhanced viscoelasticity due to added POMs. Rheology and fluorescence recovery after photobleaching (FRAP) results suggest that POM macroions not only participate in coacervate complexation but also work as cross-linkers to modulate the stimuli-responsive gel-like microstructure, yielding an optimal POM-to-polyzwitterion molar ratio for the most stable and strongest coacervate complexation. Importantly, the viscoelastic strength and mesh pore size of gel-like coacervate complexes can be tuned over several orders of magnitude by POM and salt concentrations and temperature, which offers unprecedentedly high tunability to tailor the material properties of polymer-POM coacervate complexes for practical applications.