Inflammatory bowel disease is associated with extraintestinal manifestations involving almost every organ system in the body. Crohn's disease (CD) appears to be more commonly associated with an inflammatory myopathy than ulcerative colitis. However, myopathy of the thigh in patients with CD is rare. We report an unusual site of necrotizing myositis in a patient with CD. A 23-year-old woman presented with swelling and pain at the left popliteal area that had lasted for 1 week. Twenty-two months before admission, she had presented with pyoderma gangrenosum on the left upper chest and was diagnosed with CD. A magnetic resonance imaging scan of her leg revealed diffuse swelling in the left semimembranous muscle and biceps femoris muscle that was compatible with myositis, and a cystic lesion in the distal portion of the semimembranous muscle. The findings from semimembranous muscle biopsy were also consistent with necrotizing myositis. In conclusion, myositis, although rare, can be an extraintestinal manifestation of CD.