Gliomatosis cerebri is a rare tumourous disease of the central nervous system consisting of glial cells characterized by diffuse widespread infiltration and preservation of preformed brain structures. This report describes a case of gliomatosis cerebri in a 9-year-old, male, flat-coated retriever dog with central nervous symptoms. Magnetic resonance imaging of the brain demonstrated ill-defined hyperintense zones of paramedian white matter within the right cerebral hemisphere, brainstem and cerebellum. Histological examination showed a proliferation of tumour cells in the brain structures mentioned. Glial fibrillary acidic protein-immunohistochemistry revealed a lot of fibres positively stained. Based on these findings, the alteration was diagnosed as gliomatosis cerebri.