Thirty dogs with macroscopic plasma cell tumours (PCTs) were treated with radiation therapy (RT). Twelve patients were treated with palliative-intent prescriptions (range, 4-10 Gy/fraction (median, 7 Gy/fraction) for a total dose of 20 to 35 Gy (median total dose 30 Gy). Eighteen patients received definitive-intent prescriptions (range, 3.0-4.2 Gy/fraction (median, 3 Gy/fraction) for a total dose of 42 to 54 Gy (median total dose 48 Gy). Involved sites included the oral cavity, skin, multiple myeloma (MM)-associated lytic bone lesions, bone (solitary osseous plasmacytoma; SOP), nasal cavity, larynx, retrobulbar space, lymph node and rectum. Ninety-five percent of evaluable dogs had a complete (CR; 16/22) or partial response (PR; 5/22). Patients with MM experienced significant analgesia. The median progression-free survival (PFS) was 611 days (range: 36-2001 days). Events in the non-MM cases included in-field progression (5/26, 19%) and disseminated disease (5/26, 19%). The median survival time (MST) for all dogs was 697 days (range: 71-2075 days), and when only non-MM cases were considered, MST was 771 days (range: 71-2075 days). Fourteen patients were alive without disease progression or had died of unrelated causes. Achievement of a PR was associated with an inferior PFS and MST as compared with CR. Palliative-intent RT was associated with inferior MST as compared with definitive-intent RT. RT is a useful therapeutic modality for PCTs and tumour responses are often complete and durable, with protracted survivals. The optimal radiation dose and schedule are yet to be defined.