Background: The relative efficacy and safety of more potent P2Y ₁₂ inhibitors in patients with acute coronary syndrome and high bleeding risk (HBR) undergoing percutaneous coronary intervention remains unclear. We aimed to study the treatment effect of ticagrelor and prasugrel in percutaneous coronary intervention patients presenting with acute coronary syndrome and HBR. Methods: This post hoc analysis of the ISAR-REACT 5 trial (Intracoronary Stenting and Antithrombotic Regimen: Rapid Early Action for Coronary Treatment 5) included patients with acute coronary syndrome undergoing percutaneous coronary intervention, randomized to ticagrelor or prasugrel, in whom HBR was defined as per Academic Research Consortium criteria. The primary (efficacy) end point was the composite of all-cause death, myocardial infarction, or stroke. The secondary (safety) end point was Bleeding Academic Research Consortium type 3 to 5 bleeding. Outcomes were assessed 12 months after randomization. Results: Out of the 3239 patients included in this analysis, 486 fulfilled the criteria for Academic Research Consortium-HBR definition (HBR group; ticagrelor, n=230 and prasugrel, n=256), while 2753 did not (non-HBR group; ticagrelor, n=1375 and prasugrel, n=1378). Compared with the non-HBR group, the HBR group had a higher risk for the primary (hazard ratio [HR]=3.57 [95% CI, 2.79–4.57]; P <0.001) and secondary end point (HR=2.94 [2.17–3.99]; P <0.001). In the HBR group, the primary (HR=1.09 [0.73–1.62]) and secondary (HR=1.18 [0.67–2.08]) end points were not significantly different between patients assigned to ticagrelor and prasugrel. In the non-HBR group, the primary end point (HR=1.62 [1.19–2.20]) occurred more frequently in patients assigned to ticagrelor as compared to patients assigned to prasugrel, without difference in safety (HR=1.08 [0.74–1.58]). There was no significant treatment allocation-by-HBR status interaction with respect to the primary ( P for interaction=0.12) or secondary ( P for interaction=0.80) end points. Conclusions: In patients with acute coronary syndrome undergoing percutaneous coronary intervention, HBR status increased both ischemic and bleeding risk without significant impact on the relative efficacy and safety of either ticagrelor or prasugrel. These results warrant confirmation in larger cohorts. Registration: URL: https://www.clinicaltrials.gov ; Unique identifier: NCT01944800.