Mental imagery plays a key role in the onset and maintenance of psychological disorders, and has become the target of psychological interventions for the treatment of several anxiety-related conditions. However, there are currently no transdiagnostic measures designed to assess the varied dimensions of mental imagery relevant to psychopathology. To develop and validate a new measure assessing the experiences and appraisals of negative mental imagery. The initial item pool was generated through a comprehensive literature review and interviews with subject-matter experts. An online community sample provided data for the exploratory (n=345) and confirmatory (n=325) factor analyses. The new 16-item Negative Mental Imagery Questionnaire demonstrated four subscales (Intrusiveness, Controllability, Beliefs about Mental Imagery, and Realness). Reliability and validity were good to excellent for both the full- and sub-scales. Appraisals of mental imagery captured by the new measure are consistent with previous research on mental imagery and psychopathology.