Abstract: Aspects of the social environment have been linked to the physiological mechanisms underlying behavioral self-regulation. Play, a behavior connected to regulatory behaviors such as delay of gratification and regulation of emotions, might be an aspect of social environments that is supportive of healthy physiological adaptation. We examined whether opportunities for social free play with peers, as reported by mothers, would predict children's autonomic regulation (via respiratory sinus arrhythmia; RSA) in a sample of 78 five-year-old children. As a proxy for play experience generally, frequency of social free play in the past week predicted higher levels of RSA functioning across both baseline and stress conditions, but did not account for physiological rate of change between conditions. Thus, frequent social free play opportunities might be a general positive influence on children's autonomic regulation by supporting increased parasympathetic activation but not a significant influence on children's response to stress in the moment. Attention to the role of play in autonomic regulation is critical, as children's free play opportunities might be declining.