The present invention provides a rubber modifier capable of exhibiting good dispersibility in a dispersion thereof and excellent in a rubber-modifying effect such as rubber-reinforcing effect or the likes. The invention relates to a rubber modifier comprising cellulose fibers, in which the number of the cellulose fibers having a fiber length of 450 nm or less, relative to the number of all the cellulose fibers therein, is 50% or more, to a rubber modifier dispersion comprising the rubber modifier and a dispersion medium, and to a rubber composition comprising the rubber modifier and a rubber component.