<p>Prior research has demonstrated that environmental unpredictability is associated with poor social functioning, though the mechanisms that underlie this association are largely unknown. The present study examined whether cognitive reappraisal and executive functioning mediate the longitudinal associations between unpredictability in adolescence and social connectedness in young adulthood, evaluating distinct effects on perceived and objective measures of social connectedness. The sample included 167 adolescents (47% girls) who were assessed at six time points, beginning in early adolescence (M_age = 14 years) and ending in young adulthood (M_age = 22 years). Adolescents and their respective caregivers each reported on their perceived household chaos at Time 1 (ages 14–15). Adolescents reported on emotion regulation strategy (i.e., cognitive reappraisal) and completed behavioral tasks assessing executive functioning (i.e., working memory, inhibitory control, cognitive flexibility) at Time 2 (ages 16–17), and reported on social connectedness at Time 3 (ages 21–22). The results of testing the hypothesized mediation model using structural equation modeling indicated that household chaos was statistically predictive of lower cognitive reappraisal and lower executive functioning. In turn, lower cognitive reappraisal was statistically predictive of lower social connectedness in young adulthood. Significant indirect effects were found from household chaos to lower social connectedness through less frequent cognitive reappraisal. The findings highlight the importance of adolescent cognitive reappraisal in the maintenance of young adult social connectedness. The developmental cascades from unpredictable household conditions to poor social connectedness, and implications for prevention and intervention are discussed.</p>