Given the negative health and environmental impacts of red meat consumption, reducing red meat intake in the United States is important for both human and planetary well-being. To experimentally evaluate the impact of health-focused and environment-focused messages from the Meatless Monday campaign, we conducted an online randomized experiment among US adults aged 18 years or older (n = 1244). Compared with control messages, health-focused and environment-focused Meatless Monday messages led to significantly higher perceived message effectiveness and increased intention to reduce meat consumption. (Am J Public Health. 2022;112(5):724-727. https://doi.org/10.2105/AJPH.2022.306766).