Introduction. Rectovaginal fistulas (RVFs) are abnormal connections between the rectum and vagina. Case Report. A 61-year-old female patient was admitted to the authors’ hospital with swelling, extending from the left thigh to the left lower abdomen and crepitus. An axial computed tomography scan showed air in the soft tissue of the left thigh, left buttock, perineal region, and left lower abdomen. Gas gangrene was suspected. Accordingly, the patient was administered meropenem, clindamycin, and vancomycin and underwent emergency debridement. An intraoperative examination revealed necrotizing fasciitis in the left buttock but no inflammatory signs in the thigh. On postoperative day 8, fecal matter was discharged from the patient’s vagina, and an RVF was detected by colon fiberscopy. The patient underwent resurfacing surgery with a free skin graft, and a colon stoma was fashioned 15 days after the primary surgery. The patient was discharged on day 14 following surgery with wound healing. Conclusion. The existence of free air in subcutaneous tissue combined with an infection, particularly in the extremities, is generally suggestive of gas gangrene. In the present case, subcutaneous gas was not caused by gas gangrene but rather by air inflow from an RVF. Appropriate treatment of the RVF was necessary to avoid the exacerbation of Fournier’s gangrene and prevent necrosis spreading to the thigh.