Background: Attachment and emotion regulation play a decisive role in the developmental pathways of adaptation or maladaptation. This study tested concurrent and longitudinal associations between the attachment to mother and father, sadness regulation, and depressive symptoms. Methods: A total of 1110 participants from middle childhood to adolescence completed measures of attachment, emotion regulation, and depressive symptomatology. In total, 307 of them participated in the longitudinal assessment. Results: Results revealed attachment affects emotion regulation strategies and depressive symptoms. Furthermore, we found linear effects of the cumulative number of secure attachment relationships on adaptive and maladaptive deactivating sadness regulation, as well as on depressive symptoms. Longitudinal analysis showed the significant mediating role of sadness regulation in the relationship between attachment and depressive symptoms. Conclusions: Adaptive and maladaptive deactivating sadness regulation explain the longitudinal effects of attachment on depressive symptoms. Insecurely attached children and adolescents use maladaptive and adaptive sadness regulation strategies, but differ in their hierarchy of strategy use.