Conventional cationic organobentonites often exhibit poor water compatibility, while cellulose thickeners frequently suffer from inadequate leveling properties. To address these limitations, this study innovatively utilizes Polyanionic Cellulose (PAC), a green and non-toxic anionic polymer, to modify bentonite via a hydrothermal surface modification process. The prepared PAC-modified bentonite (PAC-Mt) alleviates the incompatibility issues associated with traditional cationic modifiers. Experimental data shows that under optimized conditions (7.00% slurry concentration, 5.7% PAC dosage), PAC-Mt achieves a high apparent viscosity of 1862 mPa·s and a Shear Thinning Index (STI) of 4.1. This demonstrates a synergy where the inorganic montmorillonite provides structural stability and pseudoplasticity, while the organic PAC chains offer hydrophilicity and thickening capacity. Consequently, PAC-Mt combines the superior thickening power of cellulose with the excellent rheological properties of organoclays, establishing itself as a high-performance, eco-friendly thickening-rheological modifier for waterborne coatings.