The optimal P2Y 12 inhibitor in high bleeding risk (HBR) patients with acute coronary syndrome (ACS) remains unclear. We compared the in-hospital efficacy and safety of ticagrelor versus clopidogrel in ACS patients at HBR. We identified 22,120 hospitalized ACS patients with HBR treated with aspirin combined with either clopidogrel (n = 17,420) or ticagrelor (n = 4700) in the Improving Care for Cardiovascular Disease in China-ACS (CCC-ACS) project between November 2014 and December 2019. The median length of hospital stay was 10 days (interquartile range, 7–14 days). Compared with clopidogrel, ticagrelor was associated with a higher risk of in-hospital TIMI major or minor bleeding (4.8% vs 3.8%; adjusted OR 1.20; 95% CI 1.03–1.41; P = 0.022). The incidence of TIMI major bleeding (1.7% vs 1.1%, P = 0.005) and intracranial bleeding (0.8% vs 0.5%, P = 0.005) were also higher in the ticagrelor group than in the clopidogrel group. There was no significant difference in the rate of in-hospital major adverse cardiovascular and cerebrovascular event (MACCE) (a composite of all-cause death, myocardial infarction, stent thrombosis, or ischemic stroke) with ticagrelor compared with clopidogrel therapy (4.2% vs 4.3%; adjusted OR 1.08; 95% CI 0.90–1.28; P = 0.411). Outcomes in the propensity-matched cohorts and in sensitivity analyses were consistent with the those of the main analysis. Among ACS patients with HBR, ticagrelor as compared with clopidogrel was associated with an increased risk of in-hospital major bleeding without a significant reduction in in-hospital MACCE. https://www.clinicaltrials.gov. Unique identifier: NCT02306616. [ABSTRACT FROM AUTHOR]