Coronary sinus Reducer (CSR) implantation is currently recommended to relieve angina in patients with refractory symptoms despite optimal medical therapy and maximally achievable revascularization. The impact of diabetes mellitus on outcome after CSR implantation is at present unknown. We aimed to explore the impact of CSR in refractory angina patients with diabetes mellitus. Data from consecutive patients undergoing CSR implantation at four different centres between 2014 and 2018 were included. Patients were divided according to the presence or absence of diabetes mellitus. Primary objective of this analysis was to evaluate the clinical response to CSR implantation defined as an improvement of ≥ 1 classes of the Canadian Cardiovascular Society (CCS) Classification. A total of 219 patients were included, 116 (53%) of whom had diabetes mellitus. The median age of the population was 69 years and 167 patients (76%) were male. There were no significant differences between groups of patients with and without diabetes mellitus with respect to CCS class at baseline (p value = 0.32) and at follow-up (p = 0.75). Over a median follow-up of 393 [224–1004] days, 84 (72%) of the patients with diabetes mellitus met the primary outcome, similarly to those without diabetes mellitus (p = 0.28). Fifty-three patients (24%) did not have an improvement in CCS class and no one experienced worsening of angina. CSR implantation was equally effective in improving angina symptoms among patients with refractory angina and diabetes mellitus compared to patients without diabetes mellitus.