Nicolau syndrome (also known as embolia cutis medicamentosa and livedoid dermatitis) is a rare but severe localized adverse drug reaction of intramuscular injection of various drugs. The typical presentation is pain around the injection site soon after injection, followed by erythema, livedoid patch, hemorrhagic patch, and finally necrosis of skin, subcutaneous fat, and muscle tissue. We report a case that occurred in a 34-year-old woman after intramuscular injection of diclofenac sodium. Sonography showed diffuse thickening with increased echogenicity of the skin and subcutaneous fat layer, while MRI revealed extensive edema involving gluteal and piriformis muscles and deep fascia, and fluid collection.