This work analyzed the effect of sodium acid pyrophosphate as a rheological modifier of concentrated clay-based tailings in saline waters. The yield stress was obtained by the stress sweep method in logarithmic form using the vane-in-cup geometry. Oscillatory rheology complemented the information through amplitude and frequency sweep that correlated with the particle dispersion, evaluated by the Focused Beam Reflectance Measurement technique. Rheological properties increased with solids concentration, kaolin content, and salinity. The addition of sodium acid pyrophosphate generated decrease in all rheological parameters, following exponential decay for the yield stress case. As the proportion of clays increases, the tailings require higher reagent dosage, which was observed through changes in the characteristic dosage. The chord length distribution verified the particles’ dispersion after adding sodium acid pyrophosphate, showing fine particles release and fewer number of coarse aggregates. The tetravalent anionic phosphate molecules adsorbed on the particle’s surfaces, increasing the magnitude of the zeta potential; however, it did not occur in the same order as the rheological changes. It suggests that the dispersion mechanism is caused by both electrostatic and steric repulsion.