OBJECTIVE: Nonsuicidal self-injury (NSSI) is often cited as a key risk factor for future suicidal behavior. Capability for suicide has been repeatedly cited as an important mechanism that can account for this association. Despite this, direct tests of this hypothesis have been rare and methodologically constrained. In the present study, we conducted a direct test of this hypothesis while addressing several constraints of prior literature. METHOD: In a large sample of suicidal and self-injuring adults (n = 1,020), we tested whether changes in fearlessness about death (FAD), a core facet of the capability for suicide, accounted for the relationship between NSSI and future suicide attempts at 28-day and 2-year follow-up. FAD was assessed using the gold-standard self-report form (ACSS-FAD), an implicit test of suicide-related affect (affect misattribution paradigm-Suicide), and explicit affective ratings of suicide-relevant images. Mediation with bootstrapping was implemented to test our main hypotheses. RESULTS: As anticipated, lifetime NSSI frequency was significantly associated with suicide attempt frequency at follow-up; however, FAD failed to consistently mediate this association. Results were largely consistent across all three measures of FAD. Post hoc power analyses indicated sufficient power to detect small effects. CONCLUSIONS: Taken together, these results fail to support the hypothesis that capability for suicide explains the link between NSSI and future suicidal behavior. We discuss the implications of our results for research and theory, situating our findings in the context of recent advances in the understanding of suicide risk more broadly. (PsycInfo Database Record (c) 2021 APA, all rights reserved).