Adolescence is a critical period for social development, which COVID-19 has dramatically altered. Quarantined youths had limited in-person interactions with peers. The present study used an intensive longitudinal design to investigate changes in interpersonal dynamics and mental health during COVID-19. Specifically, we investigated whether the associations between different social contexts--that is, "spillover"--changed during COVID-19 and whether changes in social interactions during COVID-19 was associated with changes in depressive symptoms. Approximately 1 year prior to the onset of COVID-19, 139 youths reported depressive symptoms and daily interactions with parents, siblings, and friends, every day for 21 days via online questionnaires. Shortly after schools closed due to COVID-19, 115 of these youths completed a similar 28-day diary. Analyses included 112 youths (62 girls; 73% Caucasian; M[subscript age] = 11.77, range = 8 to 15 in Wave 1) who completed at least 13 diary days in each data wave. Our results show that younger adolescents experienced significant decreases in negative and positive interactions with friends, whereas older adolescents showed significant decreases in negative interactions with friends and significant increases in positive interactions with siblings. As predicted, within-day spillover of positive interactions and person-level association of negative interactions increased within the family during COVID-19, whereas within-day spillover of positive interactions between family and friends decreased. We also found a dramatic increase in depressive symptoms. More negative interactions and fewer positive interactions with family members were associated with changes in depressive symptoms. Our study sheds light on how youths' social development may be impacted by COVID-19.