The Centers for Disease Control and Prevention (CDC) HEADS UP youth sports coach materials are the most widely adopted form of concussion education for coaches across the United States-reaching millions of youth sports coaches over the last decade. These materials focus on concussion symptom identification, response, and management (e.g., return to school and sports), while also addressing the importance of communicating to athletes and their families about concussion safety. The purpose of this study was to assess the effectiveness of CDC HEADS UP materials on coach knowledge and communication with youth athletes about concussion safety. This is the first randomized control study of the CDC HEADS UP materials in real world youth sport conditions. Participants were 764 coaches at 15 YMCA associations. Cluster randomization was used to assign branches within associations to intervention (CDC HEADS UP) and control (treatment as usual) conditions. Coaches completed surveys prior to and at the end of the competitive season. Communication with athletes about concussion increased among coaches in the intervention group (adjusted risk ratio [aRR] = 1.24, 95% confidence interval [CI] = 1.14, 1.36) but not the control group (aRR = 1.09, 95% CI = 0.90, 1.31), in multi-variate analyses controlling for coach demographic characteristics and baseline communication practices. Concussion symptom knowledge and communication intentions also significantly increased in the intervention group but not in the control group. This study provides evidence that CDC HEADS UP materials increase the likelihood that youth sport coaches communicate with their athletes about concussion safety. As youth sports organizations increasingly mandate concussion education for coaches, CDC HEADS UP materials may be considered a leading resource for adoption and setting-relevant implementation.