Background <p>While associations between maternal depressive symptoms (MDS) and offspring mental health have been documented, the precise nature of this relationship, particularly potential nonlinear patterns, remains insufficiently characterized.</p> Objective <p>This study sought to examine potential nonlinear dose-response relationships between MDS and child mental health among preschool-aged children.</p> Methods <p>We conducted a cross-sectional investigation enrolling 17,115 mother-child dyads from kindergartens in a city in western China. MDS were assessed using the Center for Epidemiologic Studies Depression Scale (CES-D). Children’s mental health was assessed via maternal reports using the Strengths and Difficulties Questionnaire (SDQ). Data were collected between February 28 and March 5, 2025.</p> Results <p>Among the 17,115 mother-child dyads, 16.14% (<i>n</i> = 2,763) of mothers (mean age: 34.49 ± 4.65 years) reported elevated depressive symptoms (CES-D score ≥ 16). A nonlinear dose-response relationship was observed, with turning points at maternal CES-D scores of 17 for total difficulties and internalizing problems, 21 for externalizing problems, and 14 for prosocial behavior problems. Below these thresholds, stronger associations were observed (OR = 1.20 for total difficulties, OR = 1.16 for internalizing problems, OR = 1.10 for externalizing problems, and OR = 1.11 for prosocial behavior problems), while above the thresholds, the associations were substantially weakened.</p> Conclusion <p>This cross-sectional study identified distinctive nonlinear dose-response associations between MDS and children’s mental health outcomes in a western Chinese city. The associations were stronger below specific thresholds while weakening above these thresholds. These findings warrant further investigation and validation.</p>