Common clinical wisdom suggests that people who engage in self-injury are impulsive. However, virtually all prior work in this area has relied on individuals' self-report of impulsiveness, despite evidence that people are limited in their ability to accurately report on cognitive processes that occur outside awareness. To address this knowledge gap, we used performance-based measures of several dimensions of impulsiveness to assess whether people engaging in non-suicidal self-injury (NSSI) demonstrate greater impulsiveness than non-injurers. In Study 1, we compared adolescent self-injurers (n=64) to age-, sex-, and race/ethnicity-matched, non-injurious controls (n=30) on self-reported impulsiveness (Schedule for Affective Disorders and Schizophrenia for School Age Children, Present and LifetimeVersion) and on performance-based measures of two dimensions of impulsiveness: behavioral disinhibition (Conners' Continuous Performance Test) and risky decision-making (Iowa Gambling Task). In Study 2, we compared adult female self-injurers (n=20) with age- and race/ethnicity-matched, non-injurious controls (n=20) on self-reported impulsiveness (Barratt Impulsiveness Scale-11), and performance-based measures of behavioral disinhibition, risky decision-making, and two measures of delay discounting. In both studies, self-injurers reported greater impulsiveness; however, performance-based measures of impulsiveness failed to detect any between-group differences. We propose several potential explanations for the discrepancies observed between self-report and performance-based measures of impulsiveness and discuss directions for future research on impulsiveness and self-injury.