Background Undifferentiated pleomorphic sarcoma is a rare and highly aggressive mesenchymal tumor that lacks a specific line of differentiation. Its clinical presentation is nonspecific, and it is frequently misdiagnosed as other fibrohistiocytic tumors such as atypical fibroxanthoma, leading to delayed definitive treatment. Case presentation A 46-year-old female presented with a slowly enlarging mass on the right lower leg that had persisted for one year following a mosquito bite. Initial outpa