Background: Policies to reduce red meat intake are important for mitigating climate change and improving public health. We tested the impact of taxes and warning labels on red meat purchases in the United States. The main study question was, will taxes and warning labels reduce red meat purchases? Methods and findings: We recruited 3,518 US adults to participate in a shopping task in a naturalistic online grocery store from October 18, 2021 to October 28, 2021. Participants were randomized to one of 4 conditions: control (no tax or warning labels, n = 887), warning labels (health and environmental warning labels appeared next to products containing red meat, n = 891), tax (products containing red meat were subject to a 30% price increase, n = 874), or combined warning labels + tax (n = 866). We used fractional probit and Poisson regression models to assess the co-primary outcomes, percent, and count of red meat purchases, and linear regression to assess the secondary outcomes of nutrients purchased. Most participants identified as women, consumed red meat 2 or more times per week, and reported doing all of their household's grocery shopping. The warning, tax, and combined conditions led to lower percent of red meat–containing items purchased, with 39% (95% confidence interval (CI) [38%, 40%]) of control participants' purchases containing red meat, compared to 36% (95% CI [35%, 37%], p = 0.001) of warning participants, 34% (95% CI [33%, 35%], p < 0.001) of tax participants, and 31% (95% CI [30%, 32%], p < 0.001) of combined participants. A similar pattern was observed for count of red meat items. Compared to the control, the combined condition reduced calories purchased (−312.0 kcals, 95% CI [−590.3 kcals, −33.6 kcals], p = 0.027), while the tax (−10.4 g, 95% CI [−18.2 g, −2.5 g], p = 0.01) and combined (−12.8 g, 95% CI [−20.7 g, −4.9 g], p = 0.001) conditions reduced saturated fat purchases; no condition affected sodium purchases. Warning labels decreased the perceived healthfulness and environmental sustainability of red meat, while taxes increased perceived cost. The main limitations were that the study differed in sociodemographic characteristics from the US population, and only about 30% to 40% of the US population shops for groceries online. Conclusions: Warning labels and taxes reduced red meat purchases in a naturalistic online grocery store. Trial Registration:http://www.clinicaltrials.gov/NCT04716010. In a randomized controlled trial, Lindsey Smith Taillie and colleagues investigate the impact of taxes and warning labels on red meat purchases among US consumers. Author summary: Why was this study done?: A large body of research from tobacco, alcohol, and sugar-sweetened beverages finds that point-of-purchase policies such as warning labels and taxes reduce purchases of these products. However, little is known about whether these policies would similarly reduce purchases of red meat, an important contributor to noncommunicable disease risk and environmental damage in the United States. What did this study do and find?: This study evaluated the effects of health and environmental warning labels and taxes on red meat purchases using a randomized controlled trial in a realistic online supermarket (n = 3,518 US adults). Warning labels, taxes, and a combination of warning labels and taxes all led to modest but statistically significant reductions in red meat purchases compared to a no-policy control. The condition that included both warning labels and taxes had the largest impact on red meat purchases and also reduced calories and saturated fat purchased. Warning labels led to lower perceived healthfulness and environmental sustainability of red meat, while taxes led to higher perceived cost. The policies' impact on purchases was greatest for lower-educated and younger participants. What do these findings mean?: Implementing health and environmental warning labels and taxes could reduce red meat purchases in the US, ultimately providing health and environmental co-benefits. The main limitations were that the study differed in sociodemographic characteristics from the US population, and only about 30 to 40% of the US population shops for groceries online. [ABSTRACT FROM AUTHOR]