<p>Background: Spanking can cause adverse psychological development and biological functional changes in children. However, spanking is widely used by parents in Japan. This study explored the risk factors for family member's spanking of 3.5-year-old children using nationwide population data in Japan.</p><p>Methods: Surveys were administered to family members in Japan who had a child in 2001 (first cohort) or in 2010 (second cohort), and the data when their child was 0.5, 1.5, and 3.5 years old were used. We used multivariate binary and ordinal logistic regression analyses to examine the associations between risk factors and spanking children at 3.5 years of age, which was subcategorized into frequencies of never, sometimes, and always spanking, presented with odds ratios (ORs) and 95% confidence intervals (CIs).</p><p>Results: Among 70,450 families, 62.8% and 7.9% sometimes and always spanked their children, respectively. Children in the second cohort were spanked less frequently compared with those in the first cohort, and fathers who responded to the questionnaire spanked children less frequently than mothers who responded. Identified associated factors for spanking were male child, presence of siblings of the child, not living in a two-parent household, not living in a three-generation household, younger parents, parents with lower education, no outside work or unstable work, and lower family income.</p><p>Conclusions: We found a high prevalence of spanking and its associated factors. Approaching those with lower socioeconomic factors and promoting fathers' involvement in parenting may be important public health strategies for reducing and preventing spanking.</p>