Importance: Mental health problems are common during the transition from adolescence to young adulthood. Although perceived social support and mental health problems have been shown to be concurrently associated, longitudinal studies are lacking to document the directionality of this association, especially in emerging adulthood (late teens to late 20s). Objective: To test whether social support in emerging adulthood protects against later depression, anxiety, and suicidal ideation and suicide attempts after adjusting for a range of confounders, including prior mental health problems and family characteristics. Design, Setting, and Participants: This population-based cohort study included 1174 participants from the Quebec Longitudinal Study of Child Development. Participants underwent yearly or biennial assessment (starting from age 5 months to age 20 years). Data were collected from March 16, 1998, through June 1, 2018. Main Outcomes and Measures: Self-reported perceived social support was measured at age 19 years using the 10-item Social Provision Scale. Mental health problems, including depressive and anxiety symptoms as well as suicidal ideation and attempts, were measured at age 20 years. Social support and mental health problem raw scores were converted to z-scores to ease interpretation. Depressive and anxiety symptoms were categorized using validated cutoffs to determine clinical significance. Results: The study consisted of 1174 participants (574 female [48.89%] and 600 male [51.11%] individuals). Emerging adults with higher levels of perceived social support at age 19 years reported fewer mental health problems 1 year later, even after adjusting for a range of mental health problems in adolescence at ages 15 and 17 years (eg, depressive and anxiety symptoms and suicidal ideation and attempts) and family characteristics (eg, socioeconomic status and family functioning and structure). Higher perceived social support was associated with fewer symptoms of depression (β = -0.23; 95% CI, -0.26 to -0.18; P = <.001 and odds ratio [OR], 0.53; 95% CI, 0.42-0.66 for severe depression) and anxiety (β = -0.10; 95% CI, -0.15 to -0.04; P < .001 and OR, 0.78; 95% CI, 0.62-0.98 for severe anxiety). Higher perceived social support was associated with a lower risk for suicide-related outcomes (OR, 0.59 [95% CI, 0.50-0.70] for suicidal ideation and OR, 0.60 [95% CI, 0.46-0.79] for suicide attempts). Conclusions and Relevance: In this cohort study, emerging adults who perceived higher levels of social support reported experiencing fewer mental health problems 1 year later. These findings suggest that perceived social support may protect against mental health problems during the transition into adulthood, even in those who experience mental health problems in adolescence. Leveraging social support in prevention and treatment options may protect against mental health symptoms during this transition period.Read less <