Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive and locally invasive neoplasia with the ability to metastasize. MPNSTs are usually associated with the spinal peripheral nerves, less common cranial nerves; are histologically anaplastic and highly aggressive with local connective tissue and muscle invasion; and most require immunocytochemical and ultrastructural verification. The aim of the present case report is to describe the clinical presentation and atypical magnetic resonance imaging (MRI) findings of a nine year old cross-breed with histologically confirmed multiple MPNSTs. The dog initially presented with a two week history of ambulatory tetraparesis. An MRI showed a diffuse intramedullary lesion extending from the C3 to T3 spinal cord segments and the cerebrospinal fluid (CSF) analysis showed marked lymphocytic pleocytosis; thus, Meningomyelitis of Unknown Origin (MUO) was presumptively diagnosed. The dog was treated with an immunosuppressive tapering plan of glucocorticoids and cyclosporine. Despite significant improvement and the stabilization of the clinical signs, the dog acutely deteriorated seven months after and the MRI was repeated. Interestingly, the follow-up MRI showed multiple well-defined intradural-extramedullary mass-like lesions, affecting several spinal nerve roots along the cervicothoracic spinal cord. The histopathological examination confirmed a final diagnosis of multiple MPNSTs, and a metastatic spreading was suspected due to the presence of round cells invading the subarachnoid space. According to the reviewed literature, this is the first reported case of simultaneous MPNSTs in the cervicothoracic spinal cord of a dog. A nine year old cross-breed dog was presented with a two week history of ambulatory tetraparesis and proprioceptive ataxia affecting all four limbs. Meningomyelitis of Unknown Origin (MUO) was presumptively diagnosed based on the magnetic resonance imaging (MRI) findings and cerebrospinal fluid (CSF) analysis. The dog received a tapering dose of glucocorticoids and cyclosporine, showing significant improvement and the stabilization of the clinical signs for seven months. After this period, the dog showed an acute clinical deterioration and a follow-up MRI revealed new multiple lesions affecting different spinal nerve roots along the cervicothoracic spinal cord. Following euthanasia, a final diagnose of multiple malignant peripheral nerve sheath tumors (MPNSTs) was made based on the histopathological examination. MPNSTs can affect the cranial nerves, spinal nerves or the associated nerve roots at any location and can lead to secondary spinal cord compression. The aim of the present case report is to describe the clinical presentation and atypical MRI findings of a dog with histologically confirmed multiple MPNSTs. According to the reviewed literature, this is the first reported case of simultaneous MPNSTs in the cervicothoracic spinal cord of a dog.