We report a case of proliferative myositis arising in the pectoralis major muscle of a 59-year-old man who presented with palpable mass. The initial clinical impression was a malignant tumor. Ultrasonography revealed the lesion as a spindle-shaped hypoechoic mass, and MR imaging of the left pectoralis major muscle showed hypointensity at T1-weighted imaging, hyperintensity at T2-weighted imaging, and strong enhancement at contrast-enhanced T1-weighted imaging.