School belonging, sometimes referred to as school belonging or school connectedness, involves dimensions like positive affect towards school, relationships with teachers, and feeling socially valued. Previous research points to immediate benefits for students' mental health and wellbeing; however, evidence on the potential long-term benefits of school belonging for mental health--once young people leave the school setting--is limited. This study used data on 1568 adults from the Australian Temperament Project (ATP), a 16-wave longitudinal study which has tracked participants since infancy. The short form of the Australian Council for Educational Research (ACER) School Life Questionnaire was used to assess secondary school belonging at age 15-16 years whilst young adult mental health symptoms were evaluated using the Depression, Anxiety and Stress Scales (DASS-21) at 19-20, 23-24, and 27-28 years. Generalised Estimating Equation models were used to examine the link between secondary school belonging and mental health symptoms in young adulthood. Results showed that higher levels of all aspects of school belonging were associated with lower mental health symptoms across young adulthood ([beta] range - 0.05 to - 0.20). Associations were similar by gender. These findings underscore the importance of adolescent school belonging and in particular school status in reference to feeling socially valued, as a long-term protective factor that can mitigate against later depression, anxiety, and stress.