We present the case of a 62-year-old woman with chondromyxoid fibroma of the sphenoid sinus. Chondromyxoid fibroma is a rare bone tumor found most prevalently in long bones, so its presence at the cranial base is especially uncommon. The presence of a monoclonal gammopathy of undermined significance (MGUS) prompted consideration and investigation of a plasma cell disorder; however, CT and MRI findings followed by biopsy led to the correct diagnosis of chondromyxoid fibroma.