The invention provides a molded and crosslinked rubber material for golf balls which is obtained by molding and crosslinking a rubber composition containing rubber and rubber compounding ingredients, including a base rubber, a filler and an organic peroxide, into a spherical body of one or more layers. A monomer is impregnated into the molded and crosslinked rubber material, and one or both of the following chemical reactions is induced: (i) chemical reaction of the monomer with the rubber and/or a rubber compounding ingredient, (ii) chemical reaction of the monomer itself. The depth of impregnation by the monomer from a surface of the molded and crosslinked rubber material to the interior thereof is at least 0.3 mm. The invention also provides a method for manufacturing such a molded and crosslinked rubber material for golf balls. The invention enables the hardness profile, which is the surface hardness of the molded and crosslinked rubber material in combination with the cross-sectional hardness at given depths from the surface, to be adjusted as desired. In particular, when the molded and crosslinked rubber material is used as a golf ball core, the degree of freedom in the core hardness design, i.e., the surface and cross-sectional hardness profile of the core, can be improved.