This study describes the development and initial validation of a questionnaire that is suitable for detecting and measuring postpartum negative thoughts. Semistructured interviews with mothers who had suffered from postnatal depression were conducted to inform the content of the questionnaire. The initial questionnaire, alongside other measures, was then administered to a nonclinical sample of mothers with babies aged 0-7 months. Using principal components analysis, a two-factor structure was obtained for the Postnatal Negative Thoughts Questionnaire (PNTQ). The factors included appraisal of cognition, emotion, and situation (ACES) and baby-related and motherhood negative thoughts (BRM-NT). The psychometric properties demonstrated acceptable validity, satisfactory test-retest reliability, and internal consistency. These findings suggest that the PNTQ is a reliable and valid measure for assessing postpartum negative thoughts. Consistent with previous research, findings also suggest that appraisal of negative thoughts is more strongly related to postpartum depression than to the experience of negative thoughts per se. Clinicians may use the PNTQ to offer new mothers the opportunity to assess whether negative thoughts or metacognitive appraisals are being experienced as problematic. Additionally, a direct focus upon the metacognitive appraisals of postpartum negative thoughts may provide a useful adjunct to traditional cognitive therapy approaches. Recommendations for future research are discussed.