Pyoderma gangrenosum is an intractable disease of unknown cause involving recurrent ulcerative lesions on the skin, and may accompany ulcerative colitis, rheumatoid arthritis, leukemia, systemic lupus erythematosus, and other conditions. Here, we report a rare case of pyoderma gangrenosum in the thoracic abdomen following post-mastectomy reconstructive surgery. A 39-year-old presented at the hospital with a complaint of left papilla erosion. Skin biopsy at the site revealed invasive skin cancer, with Paget-like progression in the cancerous nipple and suspected malignancy of skin appendages. After partial mastectomy including the areola, invasive ductal breast carcinoma was diagnosed. The patient underwent a subsequent full mastectomy with simultaneous sentinel lymph node biopsy and primary breast reconstructive surgery using a rectus abdominis myocutaneous flap. Two weeks post-surgery, healing of the abdominal surgical wound was found to be delayed, and suture abscess was suspected. Despite localized treatment, an ulcerative lesion developed in the thoracic region, and pyoderma gangrenosum was diagnosed following skin biopsy. After the introduction of steroid pulse therapy, no progression of the lesion was observed. This report describes the disease characteristics, diagnosis, and treatment of post-surgical pyoderma gangrenosum and discusses the case in the context of previous literature.