We produced a Japanese translation of the 15-item myasthenia gravis (MG)-specific quality of life (QOL) scale (MG-QOL15), assessed its reliability and validity, and examined clinical factors affecting the self-perceived QOL in MG. Consecutive 327 patients with MG seen at six neurological centers were evaluated. All patients completed an MG-QOL15 Japanese version (MG-QOL15-J), the Beck Depression Inventory-Second Edition (BDI-II), and a generic health-related QOL questionnaire, the SF-36. Disease severity was determined according to the MG Foundation of America (MGFA) quantitative MG score and the MG composite. The MG-QOL15-J exhibited adequate internal reliability, test-retest repeatability, and concurrent validity with SF-36, disease severity, and known-patient groups categorized by MGFA postintervention status. Multivariate analysis revealed severity, dose of oral corticosteroids, and BDI-II as independent factors negatively affecting QOL. The MG-QOL15-J is anticipated to be a valuable clinical measure of QOL in Japanese patients with MG.