Septic arthritis is uncommon in immunocompetent young adults. It typically presents in individuals with underlying risk factors. Isolation of Group A Streptococcus (GAS) as the causative agent of septic arthritis is usually associated with autoimmune diseases, chronic skin infections or trauma. Here we report a case of a young lady who is immunocompetent without any prior history of trauma, who presented with an abrupt onset of left knee pain and swelling to the emergency department. An initial diagnosis of acute gout was made and she was treated with non-steroidal anti-inflammatory drug (NSAID). She presented again two days later to a primary care clinic with worsening knee pain and severe left calf pain. A clinical diagnosis of septic arthritis was suspected and the patient was urgently referred to the Orthopaedic team. Synovial fluid from the knee joint aspiration showed growth of GAS. A diagnosis of necrotizing fasciitis was also made as the culture taken from the left calf during incision and drainage (I&D) procedure showed a mixed growth. She eventually underwent surgical debridement twice, together with the administration of several courses of intravenous antibiotics leading to her full recovery after 45 days. This case demonstrates the challenge in making a prompt diagnosis of septic arthritis and probable Type II necrotizing fasciitis in an immunocompetent adult without underlying risk factors. Any delay in diagnosis and treatment would have increased the risk of damage to her knee joint and this may be fatal even in a previously healthy young adult. KEYWORDS: septic arthritis, immunocompetent, Group A streptococcus, necrotizing fasciitis