PTEN hamartoma tumor syndrome is a spectrum of disorders characterized by unique phenotypic features including multiple hamartomas caused by mutations of the tumor suppressor gene PTEN. Cowden syndrome and Bannayan–Riley–Ruvalcaba syndrome are representative diseases, and both have several common clinical features and differences. Because PTEN mutations are associated with an increased risk of malignancy including breast, thyroid, endometrial, and renal cancers, cancer surveillance is an important element of disease management. We report a germline mutation of the PTEN (c. 723dupT, exon 7) identified in a young woman with a simultaneous occurrence of breast cancer, dermatofibrosarcoma protuberans, and follicular neoplasm. This case suggests that it is critical for clinicians to recognize the phenotypic features associated with these syndromes to accurately diagnose them and provide preventive care.