Seroma formation is the most common complication of a modified radical mastectomy for breast cancer. Although various management or risk factors for seroma formation have been previously reported, little has been published concerning seromas with fibrous capsule formation which ultimately require a surgical resection. We herein present a case who developed a seroma with a fibrous capsule after a modified radical mastectomy for breast cancer, in spite of an uneventful intraoperative and postoperative course. The seroma was refractory to all conventional treatments, and thus finally required a surgical resection.