Deficits in identity as well as negative affect have been shown to predict self-injurious and suicidal behaviors in individuals with borderline personality disorder (BPD). However, less is known about the interactive effects of these two predictors. We examined the moderating effect of a particular component of identity, self-concept, on the relationship between negative affect and self-injurious urges utilizing ecological momentary assessments. Outpatients diagnosed with either BPD (n = 36) or any anxiety disorder but no BPD (n = 18) completed surveys throughout the day over a 21-day period. Higher levels of momentary negative affect predicted greater subsequent urges to self-injure, but only when self-concept clarity was low (z = -3.60, p < .01). This effect did not differ between diagnostic groups. The results suggest that self-concept clarity has a protective effect against self-injurious urges in light of high negative affect, and that this effect may be transdiagnostic.