There is a lack of research on the development of prosocial behavior in middle childhood. The current study addressed this gap through the application of attachment theory; attachment security has been shown to promote prosocial behavior in early childhood, and emotion regulation may be an important intervening variable in this association. A sample of 199 children (aged 6-12 years) reported on their attachment internal working models for the mother-child and father-child relationships, parents reported on child emotion regulation and emotional lability/dysregulation, and children completed a sticker donation task to assess their prosocial behavior. Child emotional lability/dysregulation served as an intervening variable in the association between father-child attachment security (communication and trust) and greater sticker donation. Mother-child and father-child attachment security was also associated with child emotion regulation, but emotion regulation was not associated with sticker donation. Findings suggest that secure attachment may foster prosocial behavior toward peers in middle childhood primarily by reducing dysregulated responses to the distress of others.