Abstract Background The association of the atherogenic index of plasma (AIP), an emerging lipid index that can predict the risk for cardiovascular disease, with adverse outcomes in type 2 diabetes mellitus (T2DM) patients with acute coronary syndrome (ACS) undergoing percutaneous coronary intervention (PCI) has not been determined. Therefore, the aim of this study was to investigate whether the AIP could independently predict adverse cardiovascular events in T2DM patients with ACS undergoing PCI. Methods This study was a retrospective analysis of a single-centre prospective registry involving 826 consecutive T2DM patients who underwent primary or elective PCI for ACS from June 2016 to November 2017. This study ultimately included 798 patients (age, 61 ± 10 years; male, 72.7%). The AIP was calculated as the base 10 logarithm of the ratio of the plasma concentration of triglycerides to high-density lipoprotein-cholesterol (HDL-C). All the patients were divided into 4 groups based on the AIP quartiles. The primary endpoint was a composite of death from any cause, non-fatal spontaneous myocardial infarction (MI), non-fatal ischaemic stroke, and unplanned repeat revascularization. The key secondary endpoint was a composite of cardiovascular death, non-fatal MI, and non-fatal ischaemic stroke. Results During a median follow-up period of 927 days, 198 patients developed at least one event. An unadjusted Kaplan-Meier analysis showed that the incidence of the primary endpoint increased gradually with rising AIP quartiles (log-rank test, P = 0.001). A multivariate Cox proportional hazards analysis revealed that compared with the lowest AIP quartile, the top AIP quartile was associated with significantly increased risk for the primary and key secondary endpoints (hazard ratio [HR]: 2.249, 95% confidence interval [CI]: 1.438 to 3.517, P < 0.001; and HR: 2.571, 95% CI: 1.027 to 6.440, P = 0.044, respectively). Conclusions A higher AIP value on admission was independently and strongly associated with adverse cardiovascular events in T2DM patients with ACS undergoing PCI.