A 61-year-old woman, a known case of diabetic nephropathy on chronic ambulatory peritoneal dialysis, presented with acute onset severe pain in the right knee, of 7 days duration. She had red hot and tender knee and was unable to bear weight. The lower half of the right thigh was swollen and tender. Right knee synovial fluid revealed 130 cells/microL (all lymphocytes). The fluid was negative for crystals and bacterial cultures. Three week later she returned with worsening of her symptoms despite receiving non-steroidal anti-inflammatory drugs and intra-articular glucocorticoid. Magnetic resonance imaging of the right thigh showed heterogeneously hyperintense right vastus medialis muscle on T2-weighted image, suggestive of diabetic muscle infarction. Therapy with analgesics and physiotherapy led to recovery in pain and thigh swelling. Diabetic muscle necrosis is a rare complication of diabetes mellitus and may present with abrupt onset of pain and loss of function of the involved extremity, mimicking septic/reactive arthritis or pyomyositis. A high index of suspicion may avoid delay in diagnosis.