Necrotizing fasciitis (NF) is a rare and often fatal soft-tissue infection involving the superficial fascial layers of the extremities, abdomen or perineum. Progression to septic shock can occur very rapidly with its associated high morbidity and mortality. NF is usually caused by beta haemolytic streptococci; less often a poly-microbial isolate is the cause. It typically occurs in patients with some degree of immune dysfunction. We present a case of severe pneumococcal necrotizing fasciitis in an obese patient with Type 2 diabetes. There was no history of trauma or evidence of diabetes-related complications. The initial presentation was with features of septic arthritis of the left knee, which subsequently progressed to NF. Differentiation from cellulitis is often difficult in the early stages. Invasive pneumococcal infections are extremely rare, with only a few reported in the literature. Moreover, our case highlights the need to consider other differential diagnoses (and to look out for complications) in patients with diabetes, especially if there is little clinical response to the initial treatment.