Nonsuicidal self-injury (NSSI) is highly stigmatized and increasingly prevalent among young adults. Perceived public NSSI stigma may hamper help-seeking, decrease social support, and impede the development of effective public health interventions to address NSSI. Research has been limited, however, by the lack of validated measures of perceived public NSSI stigma, which may be less prone to social desirability biases than other self-report measures of stigma. Thus, we adapted an existing measure of suicide attempt stigma (Scocco et al., 2012) to develop a measure of perceived public NSSI stigma (Stigma of Nonsuicidal Self-Injury [STONSI] scale). In two samples of young adult university students (one not screened and one prescreened for elevated depressive symptoms), we found support for a two-factor structure of the STONSI (one factor with reverse-coded items and one factor with non-reverse-coded items), which is used to create an average of all items, consistent with that identified for the suicide attempt stigma measure from which it was adapted. The STONSI exhibited strong internal consistency as well as within-person stability over a 6-week follow-up period. STONSI scores did not differ by lifetime history of NSSI; however, perceived public NSSI stigma was associated with indicators of NSSI severity. Future work is needed to test the validity and reliability of the STONSI in diverse samples with respect to age, clinical setting, and cultural context. If results are replicated, the STONSI may be a valuable tool for future research on NSSI stigma, its correlates, and the development of NSSI stigma interventions.