The current study developed the 60-item Multidimensional Psychological Flexibility Inventory (MPFI)—a scale assessing the 12 dimensions of the Hexaflex model. We created an exhaustive pool of 554 items including 22 of the most widely used measures from the acceptance and commitment therapy and mindfulness literatures. Exploratory and confirmatory factor analyses were used in combination with item response theory and responsiveness to change analyses in 3,040 online respondents across three studies (N_{Study 1} = 372; N_{Study 2} = 2,150; N_{Study 3} = 518) to create the MPFI. Associations between the MPFI subscales and an array of existing measures supported its convergent and discriminant validities. The MPFI offers acceptance and commitment therapy researchers new tools for elaborating treatment effects.