Background Evidence regarding the use of proton-pump inhibitors (PPIs) in COVID-19 patients remains elusive. We examined the impact of PPI use on clinical outcomes of COVID-19 patients in a territory-wide cohort. Methods We performed a retrospective cohort study using data from an electronic healthcare database managed by the Hospital Authority, Hong Kong. COVID-19 patients diagnosed virologically between 23 January 2020 and 1 January 2021 in Hong Kong were identified. The primary endpoint was a composite of intensive care unit admission, use of invasive mechanical ventilation, and/or death. PPI user was identified by PPI use within 12 months before the diagnosis of COVID-19. In subgroup analysis, current PPI users were defined as patients who used PPIs within 1 month before the diagnosis of COVID-19;past PPI users were defined as patients who used PPIs 1 to 12 months before COVID-19 diagnosis. We performed sensitivity analysis after excluding patients with short-term new NSAID use within 1 month before COVID-19 diagnosis to minimize reverse causation bias. Results We identified 8,675 COVID-19 patients (mean age 46 years, 49% male, 97.6% of all the reported cases in Hong Kong);579 (6.7%) patients had used PPI. PPI users were older, more likely to have comorbidities, concomitant medications and unfavorable laboratory parameters than non-users. Of 8,675 COVID-19 patients, 500 (5.8%) developed the primary endpoint. After propensity score (PS) balancing for patients' demographics, comorbidities, laboratory parameters, and use of medications, PPI use was not associated with the development of primary endpoint in PS weighting (weighted hazard ratio [HR] 1.11, 95% confidence interval [CI] 0.83-1.47, P=0.482) (IDDF2021-ABS- 0122 Figure 1. Cumulative incidence of primary endpoint (a composite endpoint of intensive care unit [ICU] admission, use of invasive mechanical ventilation [IMV], and death) in COVID-19 patients who were and were not proton- pump inhibitor (PPI) users after propensity score (PS) weighting in a single multiple imputation data set.), and PS matching analysis (weighted HR 0.81, 95%CI 0.57-1.14, P=0.228) (IDDF2021-ABS-0122 Figure 2. Cumulative incidence of primary endpoint (a composite endpoint of intensive care unit [ICU] admission, use of invasive mechanical ventilation [IMV], and death) in COVID-19 patients who were and were not proton-pump inhibitor (PPI) users after propensity score (PS) matching in a single multiple imputation data set). Consistent non-association was observed after multivariable adjustment (adjusted HR 0.84, 95%CI 0.66- 1.07, P=0.151), in subgroups of current and past PPI users, and in sensitivity analysis after excluding short-term new NSAID users. Conclusions PPI use is not associated with adverse clinical outcomes in COVID-19 patients. The result remains robust after PS weighting, PS matching, multivariable adjustment, and subgroup analyses.