Provided are: a rubber-reinforcing fiber, in which adhesion between a rubber and a fiber is enhanced and which is thereby capable of further improving the durability than before when used as a reinforcing material; a rubber-fiber composite; and a pneumatic tire using the same. The rubber-reinforcing fiber includes a core-sheath type composite fiber whose core portion is composed of a high-melting-point resin (A) having a melting point of 150° C. or higher and sheath portion is composed of a resin material (B) having a melting point lower than that of the high-melting point resin (A). The resin material (B) contains an olefin-based polymer (X) and a vulcanization accelerator (F).