A rubber additive composition may be capable of producing a rubber composition having well-balanced strength. Such rubber additive compositions may include: (A) anion-modified cellulose and (B) a hydrophobizing agent, wherein the hydrophobizing agent (B) includes at least one selected from the group consisting of (B1) a polyether amine having a primary amino group at an end of a polyether skeleton and (B2) an amine compound having a hydrocarbon group with a carbon number of 3 to 50 or a quaternary ammonium salt having a hydrocarbon group with a carbon number of 3 to 50.