OBJECTIVE: The aim of this study was to report the outcome and prognosis of canine patients treated medically or surgically for rectal plasmacytomas and to identify factors associated with recurrence, mortality, or progression to multiple myeloma. METHODS: The databases of 7 referral hospitals were reviewed. The Kaplan-Meier method and Cox proportional hazards analysis were used to determine the association of a range of variables with recurrence and progression-free interval for the surgically treated patients. RESULTS: 20 dogs were included. Nineteen cases were treated surgically, and 1 case was treated conservatively (analgesia and monitoring). Metastatic lesions were detected in 2 of 20 dogs (10%). Four of 19 dogs (21%) treated surgically developed postoperative complications, 1 major (1 of 19 [5.2%]) and 3 minor (3 of 19 [15.8%]). Tumor recurrence was identified in 6 of 19 dogs (31.5%). The 1-, 2-, and 3-year survival rates were 95%, 72%, and 66%, respectively. None of the variables tested were associated with occurrence of complications. Increased distance from the anus, incomplete surgical margins, and decreasing surgeon experience were associated with an increased risk of recurrence. No progression to multiple myeloma was seen in any of the cases. CONCLUSIONS: Surgical treatment of rectal plasmacytomas is associated with a low major complication rate and long survival. Metastasis and recurrence rates are significantly higher than previously reported. CLINICAL RELEVANCE: Conservative surgery leads to long-term survival, but recurrence is common. Future studies should focus on the benefits of adjuvant treatments.