Undifferentiated pleomorphic sarcomas (UPSs) are tumours of mesenchymal origin typically seen in soft tissues and histologically characterized by pleomorphic tumour cell and multinucleated giant cell (MGC) infiltration. Here, we report UPS in a 23-month-old Sprague-Dawley rat, notable for the absence of MGC infiltration and the presence of mast cells. The tumour was located in the left axillary region and was assessed using haematoxylin and eosin, Masson's trichrome, and toluidine blue staining, along with immunohistochemistry. Neoplastic cells showed pleomorphism and storiform arrangement, with no MGCs, but with significant mast cell infiltration. Vimentin positivity confirmed the mesenchymal origin. The results in this case indicate a potential role of mast cells in UPS pathology and emphasize the need for further studies on their role in tumour progression.