A rubber composition for a tennis ball includes a base rubber and a filler having a degree of flatness DL of not less than 50, the degree of flatness DL being obtained by dividing an average particle diameter D50 (μm) of the filler by an average thickness T (μm) of the filler. An amount of the filler per 100 parts by weight of the base rubber is not less than 1 part by weight and not greater than 150 parts by weight. A gas permeability coefficient GPS and a loss tangent tan δS of the rubber composition and a gas permeability coefficient GPL and a loss tangent tan δL of a rubber composition obtained by replacing the filler with kaolin clay having a degree of flatness DL of 20 satisfy (1): GPL/GPS≥1.02 and (2): |tan δL−tan δS|≤0.03. A tennis ball 2 includes a core 4 formed using the rubber composition.