Although a low resting heart rate is considered the best‐replicated biological correlate of antisocial behavior, the mechanism underlying this relationship remains largely unknown. Sensation‐seeking and fearlessness theories have been proposed to explain this relationship, although little empirical research has been conducted to test these theories. This study addressed this limitation by examining the relationship between heart rate and antisocial behavior in a community sample of 335 adolescent boys. Heart rate was measured during a series of cognitive, stress, and rest tasks. Participants also completed self‐report measures of state fear, impulsive sensation seeking, and both aggressive and nonaggressive forms of antisocial behavior. As expected, increased levels of aggression and nonviolent delinquency were associated with a low heart rate. Impulsive sensation seeking, but not fearlessness, significantly mediated the association between heart rate and aggression. This study is the first to show that impulsive sensation seeking partly underlies the relationship between aggression and heart rate, and it is one of the few to examine the mechanism of action linking heart rate to antisocial behavior. Findings at a theoretical level highlight the role of impulsive sensation seeking in understanding antisocial behavior and at an intervention level suggest it as a potential target for behavioral change.