A subperiosteal ganglion has been very rarely reported, since Ollier reported first one in 1864. A subperiosteal ganglion is produced by mucoid degeneration and cyst formation within the periosteum, which results in cortical erosion. The characteristic plain radiolographic appearance of irregular cortical erosion and scalloping, with reactive periosteal bone spicules, has been considered pathognomonic of subperiosteal ganglion. Magnetic resonance imaging is performed to further characterize the soft tissue component of the mass. As in our patient, the characteristic signal intensities of magnetic resonance imaging are especially useful in identifying the lesion as a ganglion and in defining the lobular nature and the anatomical extent. We report here the case of a 35-year-old female who had a sudperiosteal ganglion of the distal radius.