Provided are a steel cord for reinforcing rubber whose fatigue resistance is increased more than ever to enable achieving high durability that was not conventionally realized, and a pneumatic radial tire including the steel cord as a reinforcement member. In a steel cord for reinforcing rubber having a double-twist structure that includes a plurality of strands twisted together in the same direction with the same pitch and including a central structure and at least one outer layer, the central structure is composed of at least two strands being twisted around each other and each being composed of at least seven filaments being twisted together.; In a steel cord for reinforcing rubber including at least three core strands being twisted together and at least six sheath strands being twisted together around the core strands, the core strands and the sheath strands are twisted in the same direction.