Medical leech therapy is a treatment for the venous congestion of tissue flaps, grafts, and replants. We report a case of methicillin-resistant Staphylococcus aureus (MRSA) following leech application at a congested flap after mastectomy. A 45-year-old woman had an invasive ductal carcinoma. Modified radical mastectomy was performed. The chest wall defect was reconstructed with a local rotation flap. On postoperative day (POD) 1, congestion and color change were observed, and 10 medical leeches were applied to the congested area. On POD 4, another 10 medical leeches were applied. On POD 12, wound necrosis progressed and a pus-like discharge appeared. A wound swab culture revealed MRSA. Debridement was carried out on POD 15. From POD 16, vancomycin and piperacillin/tazobactam were injected for 18 days. The wound culture on POD 18 also revealed MRSA. A split-thickness skin graft was performed on POD 28. MRSA has not been clearly identified in the literature as a leech enteric bacterium. Although MRSA may have come from another source, the present case raises the possibility of MRSA infections following leech application at congested flaps. When medical leeches are applied at the congestion site of a flap, an aseptic cradle will be helpful. Vancomycin irrigation may be needed if infection occurs.