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Mast cell tumours (MCTs) are the most common cutaneous malignancy in the dog. There are few reports of MCTs affecting the vertebral column. This case series describes the clinical, histopathological and magnetic resonance imaging (MRI) findings in three dogs with metastatic MCT causing polyostotic aggressive vertebral lesions. Case 1 describes an 8-year-old male neutered small crossbreed who presented with a low grade (Kiupel) cutaneous MCT of the upper lip. Case 2 describes a 15-year-old female neutered springer spaniel with a subcutaneous MCT affecting the right caudal mammary gland. Case 3 describes a 13-year-old male neutered Nova Scotia Duck Tolling Retriever who had a history of multiple cutaneous MCTS of low and high grades (Kiupel). Following surgical excision of their respective MCTs, all patients received vinblastine and prednisolone chemotherapy. Case 1 also underwent radiotherapy. Subsequently, all patients developed spinal hyperaesthesia and paraparesis and underwent MRI. Magnetic resonance imaging revealed a variation of multifocal well-defined nodules to ill-defined patches affecting multiple vertebrae. All vertebral lesions were T2-weighted (T2W) and T1W mildly hyperintense to isointense to the spinal cord, and short tau inversion recovery (STIR) hyperintense. Where available, T1W post contrast images of the lesions showed moderate homogeneous contrast enhancement. All patients were humanely euthanised following MRI and post-mortem vertebral biopsies confirmed MCTs. To our knowledge, there is only one prior report of polyostotic MCT metastasis affecting the vertebrae. These cases, in conjunction with existing literature, demonstrate the variability of MCT metastases and suggest MCT should be considered a differential diagnosis for extradural and polyostotic vertebral lesions.