This preregistered study examined whether child temperament predicts social-emotional functioning and moderates the associations between parental distress, COVID-19–related stressors, and children’s social-emotional outcomes during the pandemic. The participants were 1,305 Finnish children (46% girls) from the FinnBrain Birth Cohort, with temperament assessed at age 2.5 years and follow-ups conducted from ages 5 to 9 years. Across five time points, parents reported their distress (anxiety and depressive symptoms) and family COVID-19–related stressors, and rated their children’s social-emotional functioning (i.e., emotional symptoms, conduct problems, hyperactivity, peer problems, and prosocial behavior) at four follow-ups (data from up to 568 children). Covariate-adjusted linear regression and mixed-effects models were used to test the main and moderating effects. Higher negative affectivity and lower effortful control in children predicted greater emotional and behavioral problems and fewer prosocial behaviors, while higher surgency was linked with fewer internalizing but more externalizing problems. Temperament also moderated the associations between COVID-19–related stressors and social-emotional functioning. Children high in negative affectivity showed stronger links to emotional symptoms early in the pandemic, whereas, unexpectedly, those low in negative affectivity exhibited more conduct problems under higher stress. Effortful control mitigated the effects of parental distress on conduct problems and hyperactivity. These findings highlight the role of temperament in influencing children’s resilience and vulnerability under prolonged stress, emphasizing the importance of identifying temperamental risk profiles for guiding future prevention and support efforts.