Lipomas are commonly encountered benign mesenchymal neoplasms composed of adipocytes that arise in the superficial subcutaneous tissues of the extremities and trunk. Occasionally, these tumors can originate from either within muscle (intramuscular) or between muscles (intermuscular). The former typically occurs within skeletal muscle of large muscles of an extremity, such as the thigh and shoulder. Intramuscular lipomas arising specifically from the pectoralis muscle are exceedingly rare. In this report, to our knowledge we describe the second case of an intramuscular lipoma arising within the pectoralis muscle. Similar to the first published report, our patient's tumor was clinically thought to be a breast mass. However, this example is unique in that it is the first of its kind to be found incidentally during screening ultrasound. Intramuscular lipomas are not routinely included in the radiologic differential diagnosis of a breast mass. However, when the tumor is posteriorly located and appears to involve the pectoralis muscle, the diagnostic possibility of intramuscular lipoma should be entertained.