In recent years, a literature has emerged describing contributions fathers make to the development of very young children. Scholars suggest that active play may be a specific area of parenting in which fathers are primary and, further, that this type of play helps children experience intense emotions and learn to regulate them. However, this hypothesis remains largely theoretical. The current study (N = 415) addresses this gap in fatherhood research by using a secondary analysis of data collected in the national Early Head Start Research and Evaluation Project (EHSREP) Fathering Substudy (Boller et al., 2006; Love et al., 2005) to examine relations between fathers' active play (measured at children's 36-month birthday-related assessment) and developmental outcomes (cognitive-social and emotion regulation) at the entry to kindergarten. Findings demonstrate that regular active physical play between fathers and young children is associated with improved developmental outcomes. However, findings support a curvilinear relationship such that moderate amounts of active play are associated with better outcomes for children, but too little or too much active play is associated with worse outcomes, especially for children with more reactive temperamental qualities. Importantly, these findings are not replicated in relation to other types of parenting activities in which fathers engage, such as reading to children or engaging at mealtime, suggesting there is a special relationship between this type of play and children's development. Furthermore, findings demonstrate that children with high emotional reactivity may benefit the most from active playtime with their fathers. These results are discussed in the context of the influence of fathering processes on child and family outcomes in low-income families.