The objective of this study was to test the Interpersonal Psychological Theory of Suicide (IPTS) proposal that the association of nonsuicidal self-injury (NSSI) with suicide attempt is mediated by acquired capability. Inpatient adolescents (n = 134) reported on suicide ideation and attempts, NSSI frequency and methods, depressive symptoms, and acquired capability for suicide. Consistent with the IPTS, both measures of NSSI were positively associated with acquired capability after accounting for depressive symptoms and past history of attempts. However, both NSSI measures explained independent variance in number of suicide attempts after controlling for suicide ideation and acquired capability. These findings contradict the IPTS and suggest that the role of NSSI in suicide attempt is mediated by variables external to the IPTS.