We herein report a rare case of oral mucormycosis following allogeneic hematopoietic stem cell transplantation. Oral mucormycosis due to Rhizopus microsporus manifested as localized left buccal mucositis with a 1-cm black focus before neutrophil recovery. Combination therapy with liposomal amphotericin B was initiated and surgical debridement was performed; however, the patient died due to progressive generalized mucormycosis. Considerable attention needs to be paid to the diagnosis and management of oral mucormycosis in post-transplant patients, thereby suggesting the importance of fully understanding the risk factors.