A new bivalve species of the genus Pinna Linnaeus, 1758, is described from shallow water off the coast of Easter Island, southeastern Pacific Ocean. Pinna rapanui sp. n. has a small, broad, slightly angulated shell with a sculpture of commarginal growth lines and prominent radial ribs decorated by almost tubular, perpendicularly erect spines. This species is the 249th marine molluscan species found in Easter Island waters, and it is, by far, the largest bivalve species living around the island. A Polynesian origin of this species is suggested; however, its definite affinities with Indo-Pacific or South American fauna are still unknown.