Research on institutional betrayal has found that institutional wrongdoing that fails to prevent or respond supportively to victims of abuse adds to the burden of trauma. In this two-study investigation with young adult university students, we demonstrated parallels between institutional betrayal and ways that families can fail to prevent or respond supportively to child abuse perpetrated by a trusted other, a phenomenon we call family betrayal (FB). In Study 1, psychometric analysis of a new FB questionnaire provided evidence of its internal consistency, unidimensionality, and convergent and discriminant validity. The majority (approximately 72%) of young adults abused in childhood reported a history of FB, with an average of 4.26 FB events (SD = 4.45, range 0-14). Consistent with betrayal trauma theory, Study 2 revealed that FB was 4x more likely to occur in relation to childhood abuse by someone very close to the victim (vs. non-interpersonal victimization), with a particularly strong effect for female participants. FB history predicted significant delay to disclosure of a self-identified worst traumatic event (ηp 2 = .017) and significant increases in dissociation (∆R 2 = .05) and posttraumatic stress (∆R 2 = .07) symptoms in young adulthood. Moreover, with FB in the regression models, only FB-not child abuse nor recent interpersonal victimization-predicted dissociation and clinically significant elevations in posttraumatic stress. Findings suggest that FB is a prevalent phenomenon among young adults abused as children and that it explains unique, clinically significant variance in posttraumatic distress, warranting increased attention from trauma researchers and clinicians.