Background: The role of left atrial (LA) function in the long-term prognosis of ST-elevation acute myocardial infarction (STEMI) is still unclear.Methods and Results: Percutaneous coronary intervention (PCI) was performed in 433 patients with the first episode of STEMI within 12 h of onset. The patients underwent echocardiography 24 h after admission. LA reservoir strain and other echocardiographic parameters were analyzed. Follow up was performed for up to 10 years (mean duration, 91 months). The primary endpoint was major adverse cardiovascular events (MACE): cardiac death or hospitalization due to heart failure (HF). MACE occurred in 90 patients (20%) during the follow-up period. Multivariate Cox hazard analyses showed LA reservoir strain, global longitudinal strain (GLS), age and maximum B-type natriuretic peptide (BNP) were the significant predictors of MACE. Kaplan-Meier curves demonstrated that LA reservoir strain <25.8% was a strong predictor (Log rank, χ2=76.7, P<0.0001). Net reclassification improvement (NRI) demonstrated that adding LA reservoir strain had significant incremental effect on the conventional parameters (NRI and 95% CI: 0.24 0.11–0.44) . When combined with GLS >−11.5%, the patients with LA reservoir strain <25.8% were found to be at extremely high risk for MACE (Log rank, χ2=126.3, P<0.0001).Conclusions: LA reservoir strain immediately after STEMI onset was a significant predictor of poor prognosis in patients, especially when combined with GLS.