Although left ventricular assist devices (LVADs) have been commonly used for patients with cardiogenic shock after acute myocardial infarction (AMI), their effects on post-AMI prognosis remain to be elucidated. In this study, we aimed to explore the effects of an LVAD on left ventricular (LV) remodeling and function at the postinfarction stage in a swine model. AMI was induced by ligation of the circumflex artery or its branches for 120 min, followed by 120 min of reperfusion. In the assist group (n = 6), LVAD was initiated at 90 min after ischemia and was maintained for support until 120 min after reperfusion, whereas the control group (n = 6) received no support. LV pressure, volume, wall stress, and stroke work were all decreased by LVAD assistance at the ischemia and reperfusion stages, and blood pressure and cardiac output were maintained. All swine were studied 1 month after the procedure, and those in the assist group showed less increased end-diastolic volumes (assist vs. control: 57.9 ± 6.6 vs. 79.0 ± 6.7 mL, P = 0.032) and sphericity (assist vs. control: 1.33 ± 0.16 vs. 1.51 ± 0.12, P = 0.01), as well as improved ejection fractions (assist vs. control: 59.0 ± 7.8 vs. 42.3 ± 6.0%, P = 0.002). Furthermore, despite a presence of a similar initial ischemic area, the percent of infarcted myocardium was reduced by 49.9% in the assist group (assist vs. control: 18.1 ± 4.8 vs. 35.3 ± 6.2%, P < 0.001). These results suggested that early assistance with an LVAD in AMI limited LV remodeling, preserved postinfarction systolic function, and improved the prognosis.