Background <p>The association between tobacco exposure and obesity remains controversial, with the potential modifying role of cadmium (Cd) unclear. Childhood population provide a critical window to assess low-dose tobacco effects. This study aimed to investigate the association between serum cotinine-assessed tobacco exposure and childhood obesity, and further evaluate whether Cd exposure acts as a potential effect modifier in this association.</p> Methods <p>This nationwide cross-sectional study investigated 5,595 school-aged children from 2017 to 2018 China National Human Biomonitoring. Serum cotinine levels were measured to assess tobacco exposure. General obesity was determined using BMI Z-score, and central obesity was assessed via waist circumference, waist-to-height ratio, and body roundness index. Weighted regression and restricted cubic splines (RCS) assessed associations between cotinine and obesity, with interaction analyses for Cd exposure.</p> Results <p>Compared to the lowest cotinine tertile, individuals with the highest tertile had higher odds of general obesity (OR = 1.485; 95% CI: 1.096, 2.011) and central obesity (OR = 1.333; 95% CI: 1.030, 1.723). RCS models indicated linear and positive associations. Cd exposure significantly attenuated the association between cotinine levels and general obesity (P_interaction = 0.002), with stronger effects observed in low Cd subgroups (OR = 1.279; 95% CI: 1.133, 1.444).</p> Conclusions <p>These results indicate positive associations of tobacco exposure with general and central obesity among nonsmoking children, and Cd acted as an effect modifier. This study suggested that environmental tobacco reduction and considering Cd co-exposure could have implications for obesity in children and adolescents.</p>