BACKGROUND Syphilis is a sexually transmitted infection (STI) caused by Treponema pallidum. If untreated, primary syphilis can progress to secondary syphilis, which has a characteristic rash and diverse systemic features. This report is of a case of secondary syphilis with mucocutaneous, articular, and pulmonary involvement. CASE REPORT A 74-year-old Moroccan man presented with an 8-week history of bilateral knee pain and swelling. On examination, he had bilateral knee effusions. Articular puncture brought an inflammatory fluid with a significant presence of white blood cells. Inflammatory markers were elevated. X-rays of both knees showed bilateral osteoarthritis with intra-articular calcification in the left knee. Nonsteroidal anti-inflammatory drugs and colchicine were prescribed, but were ineffective. A closer clinical examination of the patient revealed pigmented papules on the palms, soles, oral mucosa, trunk, and genitals. Treponema pallidum hemagglutination assay and Venereal Disease Research Laboratory results were positive in the blood (titers 1: 32) and joint fluid. A computed tomography scan of the chest revealed a focal opacity in the lateral basal segment of the right lung. The diagnosis of secondary syphilis with mucocutaneous, articular, and pulmonary involvement was made. The evolution was favorable after a single intramuscular injection of benzathine-penicillin. CONCLUSIONS Arthritis, mucocutaneous involvement, and lung lesions can be manifestations of secondary syphilis. A detailed anamnesis, clinical examination, serology, and imaging techniques are the pillars of diagnosing this condition.