Child obesity continues to be a prevalent public health issue. This meta‐analysis synthesized 17 studies investigating the association between levels of psychological stress experienced by mothers and the body mass index of their children. The overall standardized mean difference effect size was positive and significantly different from zero in cross‐sectional d = 0.20 (k = 14, 95% confidence interval [CI]: 0.06, 0.34) and longitudinal studies d = 0.18 (k = 5, 95% CI: 0.00, 0.351) and had significant heterogeneity in both (cross‐sectional, Q[13] = 193.00, P < 0.001; longitudinal, Q[4] = 29.46, P < 0.001). In longitudinal studies, effect sizes were larger when children also would have experienced the stressor, Q(6) = 4.68, P < 0.05, for toddlers than infants, Q(4) = 5.04, P < 0.05, and in higher quality studies, Q(4) = 14.58, P < 0.05. Results highlight the potential benefits of including a parent stress management component in childhood obesity prevention programmes.