Prior research has examined the relations between various facets of emotion and psychopathology, with a great deal of recent work highlighting the importance of emotion regulation strategies. Much less attention has been given to the examination of emotion reactivity. This study reports on the development and evaluation of the Emotion Reactivity Scale (ERS), a 21-item self-report measure of emotion sensitivity, intensity, and persistence, among a sample of 87 adolescents and young adults. Factor analysis revealed a single factor of emotion reactivity best characterized the data. The ERS showed strong internal consistency (* = 0.94), convergent and divergent validity via relations with behavioral inhibition/activation and temperament, and criterion-related validity as measured by associations with specific types of psychopathology and self-injurious thoughts and behaviors (SITB). Moreover, emotion reactivity statistically mediated the relation between psychopathology and SITB. These findings provide preliminary support for the ERS and suggest that increased emotion reactivity may help explain the association between psychopathology and SITB.