The under-reporting of concussion symptoms by adolescent athletes is a serious concern. There is a need to review the under-reporting literature, identify successful methods for improving reporting behaviours and to inform directions for future interventions and educational programming. Therefore, the purpose of this systematic review is to examine and synthesize the current literature on sport-related concussion under-reporting in adolescent athletes. The review addresses the following two objectives: 1) What is the evidence for sport-related concussion under-reporting by adolescent athletes; and 2) What are the barriers or facilitators that contribute to under-reporting? Consideration is also given to determine whether concussion under-reporting behaviours differ between male and female athletes, age groups, or type of sport. Six electronic bibliographic databases were searched for eligible articles, and studies were reviewed at the title/abstract level, followed by independent full-text screening. Overall, 5391 articles were screened at the title abstract level, and 205 were eligible for full-text review. Full-text screening and data extraction are ongoing, and study quality was assessed used a mixed methods assessment tool. Results are analyzed using a narrative synthesis approach; this framework takes a non-linear approach to explore, analyze and synthesize the results from the included studies. Findings consider sociocultural aspects of sport that may influence under-reporting behaviours in young athletes, including athlete identity, risk-taking tendencies and normative sport culture.