A 78-year-old male patient was admitted to our hospital due to a soft mass with a rapid growth over 10 d. In 2020, he accidentally noticed a soft mass the size of a pigeon egg on the inner side of his left knee joint with good mobility and no local redness, swelling or pain. He visited the local clinic, and Color Doppler ultrasound examination suggested that the tumor was a pilomatricoma. No further treatment was performed at that time. However, when the tumor increased over 10 d, the patient felt occasional numbness and discomfort in the left lower extremity. The patient was previously diagnosed with a superficial spreading melanoma on his left thigh, which was excised in our hospital in 2015. The postoperative positron emission tomography-computed technology examination showed no evidence of regional lymph node metastasis and distant metastasis, so the subsequent radiotherapy or chemotherapy was not performed. The patient had no relevant family medical history. A painless, tough tumor 5 cm × 5 cm × 4 cm in diameter within his left knee joint with a clear boundary, rough surface and hot flushed skin was noted. There were no obvious abnormalities in sensation, blood supply and movement of the left lower limb. Before operation, the patient’s erythrocyte sedimentation rate increased (30 mm/h, normal range: 0-15 mm/h) and C-reactive protein was slightly high (10.3 mg/L, normal range: 0-10 mg/L). Other laboratory examination results were normal, including hematological, coagulation, kidney and liver functions as well as electrolytes. Color Doppler ultrasonography revealed a hypoechoic mass with an unclear boundary in the subcutaneous soft tissues of the medial left knee with abundant dotted and band-shaped blood flow signals in and around the lesion. Computed tomography scanning showed a subcutaneous lesion in the left medial femoral area with an average computed tomography value of 34 HU, which was significantly lower than the adjacent soft tissue (62 HU). The skin adjacent to the tumor was slightly thickened, and no obvious signs of bone destruction were observed. Magnetic resonance imaging (MRI) examination revealed a subcutaneous and lobulated tumor, measuring 4.51 cm × 2.75 cm × 3.00 cm in maximum size, irregular in shape but well-circumscribed with a rich blood supply and the absence of a visible entering or exiting nerve. The lesion was heterogeneously hypointense on T1-weighted images (T1WI) and fat-saturated T2-weighted images (T2WI) in the sagittal plane. It also showed heterogeneous enhancement with nonenhanced focal areas on contrast-enhanced T1WI in the sagittal and coronal plane. The hyperintense grid-like fascia on fat-saturated T2WI and enhanced thickened skin on contrast-enhanced T1WI suggested peritumoral edema and the invasion of surrounding soft tissues.