BACKGROUND: Spinal neoplasms are sparsely documented in juvenile dogs. Case reports and small case series have described nephroblastomas, primitive neuroectodermal tumours, gliomas, certain sarcomas, and osteochondromas, but round cell sarcomas have not previously been documented. CASE PRESENTATION: This case report describes a 10-week-old female Rottweiler puppy with acute onset of progressive ataxia and pelvic limb lameness. Neurological examination localised a T3-L3 myelopathy and MRI revealed an ovoid, well-marginated mass extending from mid L3 to caudal L4 vertebrae. Post-mortem examination, histopathology, and immunohistochemistry confirmed a round cell sarcoma of extradural origin. CONCLUSION: Our case report stresses the importance of performing MRI even in very young individuals with acute progressive signs of spinal cord lesions. Clinicians should include spinal tumours as a differential diagnosis for juvenile canines with spinal neurological signs. Round cell sarcoma should be added to the list of spinal tumours in young dogs.