Background: Diabetes mellitus (DM) plays a key role in the pathophysiology of metabolic syndrome (MetS). This study aimed to investigate the association among DM, low-attenuation plaque (LAP) volume, and cardiovascular outcomes across metabolic phenotypes in patients with suspected coronary artery disease (CAD) who underwent coronary computed tomography angiography (CCTA). Methods: We included 530 patients who underwent CCTA. MetS was defined as the presence of a visceral adipose tissue area \ge100 cm{}^2 in patients with DM (n = 58) or two or more MetS components excluding DM (n = 114). The remaining patients were categorised as non-MetS patients with DM (n = 52) or without DM (n = 306). A CCTA-based high-risk plaque was defined as a LAP volume of >4%. The primary endpoint was the presence of a major cardiovascular event (MACE), which was defined as a composite of cardiovascular death, acute coronary syndrome, and coronary revascularization. Results: The incidence of MACE was the highest in the non-MetS with DM group, followed hierarchically by the MetS with DM, MetS without DM, and non-MetS without DM groups. In the multivariable Cox hazard model analysis, DM as a predictor was associated with MACE independent of LAP volume >4% (hazard ratio, 2.68; 95% confidence interval, 1.16–6.18; p = 0.02), although MetS did not function as an independent predictor. A LAP volume >4% functioned as a predictor of MACE, independent of each metabolic phenotype or DM. Conclusions: This study demonstrated that DM, rather than MetS, is a predictor of coronary events independent of high-risk plaque volume in patients who underwent CCTA.