Importance: The COVID-19 pandemic continues to pose unprecedented challenges to worldwide health. While vaccines are effective, supplemental strategies to mitigate the spread and severity of COVID-19 are urgently needed. Emerging evidence suggests susceptibility to infections, including respiratory tract infections, may be reduced by probiotic interventions; therefore, probiotics may be a low-risk, widely implementable modality to mitigate risk of COVID-19 disease, particularly in areas with low vaccine availability and/or uptake. Objective: To determine whether daily probiotic Lactobacillus rhamnosus GG (LGG) is effective in preventing development of symptoms of illness within 28 days of COVID-19 exposure. Design: This randomized, double-blind, placebo-controlled trial across the United States (PROTECT-EHC) enrolled in 2020-2021. Participants were followed for 60 days. Setting: Describe the study setting to assist readers to determine the applicability of the report to other circumstances, for example, multicenter, population-based, primary care or referral center(s), etc. Participants: Participants included individuals > 1 year of age with a household contact with a recent (≤ 7 days) diagnosis of COVID-19. 182 participants were enrolled and randomized during the study period. Intervention: Participants were randomized to receive daily oral LGG or microcrystalline cellulose placebo for 28 days. Main Outcomes and Measures: The primary outcome was development of symptoms within 28 days of exposure to a COVID-19-infected household contact. Stool was collected to evaluate for changes in microbiome structure. Results: 182 participants were enrolled and randomized during the study period. Intention-to-treat analysis showed that participants randomized to LGG were less likely to develop symptoms versus those randomized to placebo (26.4% vs. 42.9%, p=0.02). Further, LGG was associated with a statistically significant reduction in COVID-19 diagnosis (log rank p=0.049) via time-to-event analysis. Overall incidence of COVID-19 diagnosis did not significantly differ between LGG and placebo groups (8.8% vs. 15.4%, p=0.17). LGG was well-tolerated with no increased side effects versus placebo. Placebo recipients were more likely to stop the study product, temporarily or permanently, due to symptoms attributed to the study product (5.5% vs. 0%, p = 0.02). Conclusions and Relevance: Our study suggests that LGG is well-tolerated and is associated with prolonged time to development of COVID-19 infection, reduced incidence of symptoms, and changes to gut microbiome structure when used as post-exposure prophylaxis within 7 days after exposure. This preliminary work may inform the approach to prevention of COVID-19, particularly in underdeveloped nations where Lactobacillus probiotics have already been