An 8-y 9-mo-old male Pug dog was presented because of anorexia, hindlimb ataxia, vomiting, and progressive weight loss. Clinical examinations revealed atrophic hindlimb muscles with decreased postural reaction, enlargement of the cardiac silhouette and megaesophagus on radiograph, and reduced cardiac contractility on ultrasonography. The dog died 10 d after the initial examination, and an autopsy was performed. Grossly, the heart was enlarged, with multifocal-to-coalescing extensive plaque-like areas of discoloration on the epicardial surface. On cross-section, the ventricles were moderately dilated, and discoloration extended into the myocardium. Peripheral lymph nodes were of normal size. Histologically, atypical lymphoid cells replaced the myocardium and also extensively infiltrated peripheral nerve bundles in various organs. With immunohistochemistry, the neoplastic cells exhibited strong immunoreactivity for CD3 and CD20, and were negative for CD8, granzyme B, CD79α, and Pax5. Double-label immunofluorescence confirmed co-expression of CD3 and CD20 by the neoplastic cells. Molecular clonality analysis presented a clonal T-cell receptor gamma gene rearrangement. The case was diagnosed as a CD3+/CD20+ peripheral T-cell lymphoma with prominent cardiac and peripheral nerve involvement, indicating neurolymphomatosis.