BACKGROUND: The burden of adverse health effects from heat exposure is substantial, and outdoor workers who perform heavy physical work are at high risk. Though heat prevention interventions have been developed, studies have not yet systematically evaluated the effectiveness of approaches that address risk factors at multiple levels. OBJECTIVE: We sought to test the effectiveness of a multi-level heat prevention approach (heat education and awareness tools [HEAT]), which includes participatory training for outdoor agricultural workers that addresses individual and community factors and a heat awareness mobile application for agricultural supervisors that supports decisions about workplace heat prevention, in the Northwest United States. DESIGN: We designed the HEAT study as a parallel, comparison, randomized group intervention study that recruited workers and supervisors from agricultural workplaces. In intervention arm crews, workers received HEAT training, and supervisors received the HEAT awareness application. In comparison arm crews, workers were offered non-HEAT training. Primary outcomes were worker physiological heat strain and heat-related illness (HRI) symptoms. In both worker groups, we assessed HRI symptoms approximately weekly, and heat strain physiological monitoring was conducted at worksites approximately monthly, from June through August. DISCUSSION: To our knowledge, this is the first study to evaluate the effectiveness of a multi-level heat prevention intervention on physiological heat strain and HRI symptoms for outdoor agricultural workers. TRIAL REGISTRATION: ClinicalTrials.gov Registration Number: NCT04234802.