Introduction: Nonsuicidal self-injury (e.g., self-cutting without lethal intent) is a widely occurring behavior among adolescents and emerging adults. Heightened impulsivity during the adolescent and emerging adult years may contribute to an increased risk for nonsuicidal self-injury onset and engagement during these developmental periods; however, longitudinal research on impulsivity and nonsuicidal self-injury among young persons is lacking. Method: To extend previous research, and elucidate the direction of effects between impulsivity and nonsuicidal self-injury, 782 emerging adults (75% female) completed assessments of impulsivity and nonsuicidal self-injury annually for three years. Results: Path analysis revealed a bidirectional relation, such that higher impulsivity predicted greater nonsuicidal self-injury frequency over time, and greater nonsuicidal self-injury frequency predicted higher impulsivity over time, even after taking into account past scores on these variables as well as other participant factors (e.g., age, sex, depressive symptoms, anxiety). Gender did not moderate the pattern of associations. Conclusions: The present findings indicate that impulsivity may not only lead to nonsuicidal self-injury, but that frequent engagement in nonsuicidal self-injury may undermine regulatory skill development and lead to greater impulsivity over time during the emerging adult years.