Pyogenic tenosynovitis occurs almost exclusively in the flexor tendons of distal extremities, more commonly in the hand/wrist than the ankle/foot. Most commonly documented in the literature of the rarer extensor pyogenic tenosynovitis are case reports in the upper extremities caused by atypical bacteria or fungi, with only two cases caused by Staphylococcus aureus. It is rare for isolated tenosynovitis to occur in the extensor tendons of the lower extremity in a patient with no known trauma, IV drug use, or significant comorbidities. We report a case of a 22-year-old male who presented with a two-day history of progressive dorsolateral foot erythema, swelling, and pain. He denied any history of trauma or evidence of foot wounds, abrasions, or punctures. His examination and ultrasound were consistent with extensor tenosynovitis of the extensor digitorum longus. He was treated with intravenous antibiotics and surgical irrigation and debridement. Intraoperatively, a large phlegmon was identified in the tenosynovium. His symptoms resolved postoperatively, and he made a full recovery with no deficits. Pyogenic extensor tenosynovitis warrants consideration in the differential diagnosis of patients presenting with isolated dorsolateral foot erythema, swelling, and pain, despite no history of trauma or intravenous drug use.