A rare case of spindle cell lipoma of the knee in a 58-year-old woman is presented. A soft tissue mass on the lateral aspect of the knee, measuring 5 x 6 cm in size, that had been noticed 1 year previously showed slightly lower signal intensities both on T1- and T2-weighted magnetic resonance images than those of subcutaneous adipose tissue. Because pathological findings of the tiny specimen obtained by needle biopsy showed lipogenic tumor and the possibility of well-differentiated liposarcoma could not be ruled out, wide excision was performed. Histopathological examination revealed that the tumor consisted of spindle cells, collagen fibers, and lipocytes. In addition, immunohistochemical study showed positive staining for CD34. From these histological findings, diagnosis of spindle cell lipoma was made. Although expected sites of spindle cell lipoma are the posterior neck, shoulder region, and upper back, it may also arise in the lower extremity. Therefore, when radiological findings suggest lipogenic tumor but are different from those of lipoma, spindle cell lipoma as well as well-differentiated liposarcoma should be considered for differential diagnoses.