Objective To investigate the association between maternal depressive symptoms and adolescent suicidal ideation, and to examine the chain mediating roles of childhood trauma and ineffectiveness. Methods A cross-sectional online survey was administered by school psychologists to 4 157 mother-adolescent pairs from middle schools in Shanghai and Henan, China. Measures included the Center for Epidemiological Studies Depression Scale, the Childhood Trauma Questionnaire, and the Children's Depression Inventory. Using Bootstrap method to examine the chain mediating effect of childhood trauma and ineffectiveness on the relationship between maternal depression symptoms and adolescent suicidal ideation. Results The prevalence of maternal depressive symptoms was 17.68% (735/4 157); among adolescents, the prevalence of depressive symptoms was 15.49% (644/4 157), and suicidal ideation was 28.19% (1 172/4 157). Adolescent depressive symptoms and suicidal ideation were positively correlated with maternal depressive symptoms, childhood trauma, and ineffectiveness (all P<0.01). Childhood trauma significantly mediated the association between maternal and adolescent depressive symptoms (95%CI: 0.046 9-0.077 2). The chain mediation of childhood trauma and ineffectiveness in the association between maternal depressive symptoms and adolescent suicidal ideation was also significant (95%CI: 0.000 7-0.001 3). Conclusions Higher maternal depressive symptom levels are associated with a greater likelihood of adolescents' exposure to childhood trauma, which increases adolescents' ineffectiveness and, in turn, is associated with suicidal ideation. This chain effect has important implications for social interventions targeting adolescent depression.