Provided is a fibrillated chemically modified cellulose fiber, which has a type-I cellulose crystallinity of at least 50%, an anionic charge density of 0.10-2.00 meq/g, and an average fiber diameter of greater than 500 nm. Also, provided is a fibrillated chemically modified cellulose fiber, wherein the value (A/B) obtained by dividing the viscosity A measured at a shear rate of 0.01/sec by the viscosity B measured at a shear rate of 1000/sec in an aqueous dispersion having a solid content of 1 mass % is at least 100. These fibers have high water retention and high thixotropy.