There is a growing interest in the optimizing care of acute coronary syndrome (ACS) in young patients, largely owing to their potential for longer life expectancy. Herein, we aimed to investigate the clinical characteristics and outcome of young ACS patients (e.g. under 60 year old) from a Japanese multicenter percutaneous coronary intervention (PCI) registry (KiCS-PCI). KiCS-PCI registered consecutive ACS patients from 15 institutions, and 1560 (24.0%) out of 6499 ACS-related PCI involved patients aged <60 years. In this group, prevalence of dyslipidemia, smoking and family history of premature coronary artery disease (CAD) was higher, while the other classical risk factors were lower when compared to the old patients. After adjustment for known confounders, presentation with cardiogenic shock (CS) before PCI (OR 32.57, 95% CI 12.06–87.97), culprit lesion of LMT (OR 7.53, 95% CI 1.26–44.98), multi-vessel disease (OR 3.82, 95% CI 1.37–10.63) and higher body mass index (OR 1.12, 95% CI 1.00–1.24) showed association with higher in-hospital mortality in young patients. Multi-vessel disease (OR 4.1, 95% CI 1.9–8.9) and chronic kidney disease (OR 3.56, 95% CI 2.26–5.68) were associated with CS presentation. CS presentation was inversely associated with classical risk factors such as hypertension (OR 0.61, 95% CI 0.38–0.96), family history of CAD (OR 0.49, 95% CI 0.25–0.96), and dyslipidemia (OR 0.45, 95% CI 0.29–0.71) and culprit lesion of RCA (OR 0.60, 95% CI 0.37–0.94). Overall, ACS in the younger population was observed frequently, accounting for a quarter of ACS-related PCI. CS was a harbinger for in-hospital mortality in these patients.