Background The efficacy and safety of ticagrelor versus prasugrel in patients with acute coronary syndrome and prior myocardial infarction (MI) remain unstudied. We aimed to assess the treatment effect of ticagrelor versus prasugrel according to prior MI status in patients with ACS. Methods and Results Patients with acute coronary syndrome planned for an invasive strategy and randomized to ticagrelor or prasugrel in the ISAR-REACT (Intracoronary Stenting and Antithrombotic Regimen: Rapid Early Action for Coronary Treatment) 5 trial were included. The primary end point was the composite of 1-year all-cause death, MI, or stroke; the secondary safety end point was the composite of 1-year Bleeding Academic Research Consortium type 3 to 5 bleeding. The study included 4015 patients (prior MI=631 patients; no prior MI=3384 patients). As compared with patients without prior MI, the primary end point occurred more frequently in patients with prior MI (12.6% versus 7.2%; hazard ratio [HR], 1.78 [95% CI, 1.38-2.29]); the secondary safety end point appears to differ little between patients with and without prior MI (5.8% versus 5.7%, respectively; HR, 1.02 [95% CI, 0.71-1.45]). With regard to the primary end point, ticagrelor versus prasugrel was associated with an HR of 1.62 (95% CI, 1.03-2.55) in patients with prior MI and an HR of 1.28 (95% CI, 0.99-1.65) in patients without prior MI (Pint=0.37). With regard to the secondary safety end point, ticagrelor versus prasugrel was associated with an HR of 1.28 (95% CI, 0.56-2.91) in patients with prior MI and an HR of 1.13 (95% CI, 0.82-1.55) in patients without prior MI (Pint=0.79). Conclusions Patients with acute coronary syndrome and prior MI are at higher risk for recurrent ischemic but not bleeding events. Prasugrel is superior to ticagrelor in reducing the risk of ischemic events without a tradeoff in bleeding regardless of prior MI status. Registration URL: https://www.clinicaltrials.gov; Unique identifier: NCT01944800.