A primary intracerebral plasmacytoma was identified in a 7-year-old spayed female Boston Terrier. Grossly, a well-demarcated, 2 cm in diameter, roughly spherical tumor was in the rostral aspect of the left cerebral hemisphere. Histologically, the neoplasm was composed of sheets of round cells with distinct plasmacytoid features and marked anisocytosis and anisokaryosis. Cells were positive for vimentin, CD18, CD79a, and lambda light-chain, and negative for kappa light chain, cytokeratin, lysozyme, glial fibrillary acidic protein, and S100 protein. Clonally rearranged B-cell antigen receptor genes were detected by PARR (polymerase chain reaction for antigen receptor rearrangements), confirming clonal proliferation of B lymphocytes. Although primary solitary intracerebral plasmacytoma is rare in dogs and other species, it should be included in the differential diagnosis for central nervous system round-cell neoplasms. Clonality testing can be utilized to support the histological diagnosis of this neoplasm type.