Nicolau syndrome is rare cutaneous adverse reaction following injection of various drugs. The disease has been associated with almost all classes of medications, such as nonsteroidal anti-inflammatory drugs, local anesthetics, corticosteroids, antibiotics, vitamin B complexes, antihistamines, and vaccines. The most severe cases of this complication occur after injection of nonsteroidal anti-inflammatory drugs. Subcutaneous injection is an important factor in the pathogenesis of the syndrome. We report a case of Nicolau syndrome resulting from intramuscular injection of diclofenac and discuss some precautions to prevent this complication.