A sulfated cellulose fiber having a cellulose I crystal structure is provided.A chemically modified cellulose fiber which has a cellulose I crystal and in which some hydroxyl groups of cellulose are substituted with a substituent represented by formula (1). An amount of the substituent introduced is 0.1 mmol to 3.0 mmol per 1 g of the chemically modified cellulose fiber, and an average degree of polymerization is 350 or more. (In formula (1), M represents a monovalent to trivalent cation.) In the production of the chemically modified cellulose fiber, a cellulose fiber is treated with sulfamic acid while maintaining a cellulose fiber shape to allow sulfamic acid and a cellulose fine fiber which is a constituent of the cellulose fiber to react with each other.
