Using a natural dispersant to prepare a well-dispersed aqueous clay suspension with remarkable fluidity and stability is of significance because of the gradual depletion of fossil-based raw materials. In this study, nanofibrillated cellulose (NFC) prepared by 2,2,6,6-tetramethylpiperidine-1-oxyl oxidation method was used as a green dispersing agent to effectively disperse clay particles in water. The carboxylated NFC dispersant increased the maximum solid content of an aqueous clay suspension while reducing its static viscosity. As compared to a sodium polyacrylate-dispersed clay suspension, the NFC-dispersed clay suspension not only exhibited similar maximum solids content, particle-size distribution, and rheology, it also had better water retention value and stability. In addition, we found that the increased carboxyl content of NFC had a significant positive impact on the dispersing effect of clay suspensions. This green and effective carboxylated NFC shows its potential to disperse aqueous clay particles with excellent performance for advanced uses.