Abstract Background and Aims The efficacy and safety of preventive percutaneous coronary intervention (PCI) for treating vulnerable plaques in diabetic patients remain unclear. Methods The PREVENT (Preventive Coronary Intervention on Stenosis with Functionally Insignificant Vulnerable Plaque) trial was a randomized clinical trial that compared preventive PCI plus optimal medical therapy with optimal medical therapy alone in patients with non-flow-limiting (fractional flow reserve >0.80) vulnerable plaques identified via intracoronary imaging. Randomization was stratified by diabetes status. The primary endpoint was a composite of cardiac death, target-vessel myocardial infarction, ischaemia-driven target-vessel revascularisation, or hospitalization for unstable or progressive angina at 2 years. Results Among 1606 randomized patients, 490 (30.5%) had diabetes. Diabetic patients underwent PCI for non-target lesions before randomization more frequently than non-diabetics (40.6% vs. 33.8%, P = .009). There were no significant differences in the incidence of the primary endpoint between diabetic and non-diabetic patients [1.8% vs. 1.9%; hazard ratio 0.98; 95% confidence interval 0.45–2.14); P = .956]. However, the primary endpoint at 2 years was less frequent with preventive PCI compared with optimal medical therapy alone in both diabetic (0% vs. 3.7%; P = .004) and non-diabetic patients (0.5% vs. 3.2%; hazard ratio 0.16; 95% confidence interval 0.05–0.55; P = .004), without a significant interaction between diabetic status and randomized strategy. Conclusions The risk of adverse clinical events was similar between diabetic and non-diabetic patients with non-flow-limiting vulnerable coronary plaques. However, preventive PCI was associated with a lower incidence of the primary endpoint at 2 years, regardless of diabetes status.