This research presents a multidimensional conceptualization of unforgiveness and the development and validation of the unforgiveness measure ( UFM ). The scale was developed based on a qualitative study of people's experiences of unforgiven interpersonal offences (Study 1). Three dimensions of unforgiveness emerged (Study 2): emotional‐ruminative unforgiveness, cognitive‐evaluative unforgiveness, and offender reconstrual. We supported the scale's factor structure, reliability, and validity (Study 3). We also established the convergent and discriminant validity of the UFM with measures of negative affect, rumination, forgiveness, cognitive reappraisal, and emotional suppression (Study 4). Together, our results suggest that the UFM can capture variability in victims’ unforgiving experiences in the aftermath of interpersonal transgressions. Implications for understanding the construct of unforgiveness and directions for future research are discussed.