An unusual malignant spindle-cell sarcoma in the right atrium of a 7-year-old male hunting terrier is described. The neoplasm arose from the endocardium, protruded into the lumen of the right atrium, was covered with endothelium, and showed local invasive growth into the atrial wall. The tumour was composed of interlacing bundles of spindle cells, sometimes arranged in whorls resembling the Antoni type A pattern. The extracellular matrix showed abundant reticulin fibres. Immunohistochemistry revealed an intense labelling of tumour cells for vimentin, and a partial labelling for neuron-specific enolase, S100 protein, and Melan-A. The morphology of the tumour indicated a primary malignant peripheral nerve sheath tumour, resembling a neoplasm described in rats.