Background <p>Obesity and metabolic syndrome are related to cardiovascular events. However, the association different metabolic phenotypes and obesity with cardiovascular events among young adults aged 18–45 with acute coronary syndrome (ACS) remains unclear. The study aimed to investigate the prognosis of patients aged 18–45 years with ACS based on their metabolic phenotype.</p> Methods <p>This study included 1787 patients with ACS ≤ 45 years of age who underwent coronary angiography. Patients were divided into four groups according to metabolic phenotype: metabolically healthy non-obesity (MHN); MHO; metabolically unhealthy non-obesity (MUN); and metabolically unhealthy obesity (MUO). The primary outcome was major adverse cardiovascular events (MACE), including all-cause death, myocardial infarction, stroke, or unplanned revascularization.</p> Results <p>Among 1787 patients with ACS, the median age was 41.6 years, 1675 (93.7%) were men, 1111 (62.2%) had obesity, and 659 (36.9%) were classified as MHO. During a median 65 months follow-up, 404 MACE occurred. Multivariate analysis showed that MHO was correlated with a decreased risk of MACE, while MUN significantly increased the risk compared to MHN (MHO: HR 0.69, 95%CI 0.52–0.92, <i>P</i> = 0.011; MUN: HR 1.47, 95%CI 1.07–2.02 <i>P</i> = 0.018). Moreover, restricted cubic spline analysis revealed a linear relationship between body mass index (BMI) and the incidence of MACE (<i>P</i>_nonlinear= 0.304, <i>P</i>_overall &lt; 0.001).</p> Conclusions <p>MHO was correlated with a decreased risk of MACE, while MUN significantly increased the risk compared to MHN. Moreover, there was a linear relationship between BMI and the incidence of MACE.</p>