Primary brain tumours in cetaceans are rare with only four reported cases of intracranial tumours in the scientific literature. A juvenile female, striped dolphin live-stranded at Whitepark Bay, Co Antrim, Northern Ireland, UK, and died after an unsuccessful attempt at refloatation. Necropsy examination revealed a large, soft, non-encapsulated friable mass, which expanded and replaced the frontal lobes, corpus callosum and caudate nucleus of the brain and extended into the lateral ventricles, displacing the thalamus caudally. Microscopically, this comprised moderately pleomorphic neoplastic cells arranged variably in dense monotonous sheets, irregular streams, ependymal rosettes, 'ependymoblastomatous rosettes' and multilayered to pseudostratified tubules. Liquefactive necrosis, palisading glial cells, haemorrhage and mineralization were also observed. Immunohistochemically, the neoplastic cells expressed vimentin but not S100, glial fibrillary acidic protein, cytokeratin, neuron-specific enolase or synaptophysin. Based on these findings a diagnosis of primitive neuroectodermal tumour was made. Monitoring and recording such cases is crucial as neoplasia may be related to viral, carcinogenic or immunosuppressive chemical exposure and can ultimately contribute to assessing the ocean health.