This study aimed to clarify the association between mothers' problematic Internet use (PIU) and the thinness of their children. We analyzed data collected from health examinations of young children aged 4 months, 1.5 years, and 3 years of age performed in Matsue city, Japan, between April 2016 and March 2017. The subjects comprised 1,685 (866 boys, 819 girls) children aged 4 months, 1,728 (898 boys, 830 girls) aged 1.5 years, and 1,672 (802 boys, 870 girls) aged 3 years. Logistic regression analysis was used to clarify the association between mothers' PIU (Young's Diagnostic Questionnaire for Internet Addiction score: ≥4) and the thinness (body mass index: <15) of their children after adjusting for covariates such as birth weight, nutritional form, parental smoking status, maternal age, skipping breakfast, eating snacks, sleeping late, outdoor play, and daytime caregiver. Analysis after stratification by sex and age revealed that the mothers' PIU was significantly associated with their children's thinness only in boys aged 4 months or 1.5 years (odds ratio [OR] = 3.16, 95% confidence interval [CI] = 1.00-9.96 and OR = 2.68, 95% CI = 1.04-6.89, respectively). Mothers' PIU may promote thinness among boys aged <3 years. As the nutritional status of children aged <3 years is affected by maternal feeding attitudes, our findings suggested that mothers who exhibit PIU do not provide adequate care for their children, particularly regarding feeding. In contrast, no association between mothers' PIU and their children's thinness was observed in girls.