The construct of psychological flexibility (PF) is a central concept in acceptance and commitment therapy. It is defined as the process of contacting the present moment fully as a conscious human being and persisting in or changing behavior in the service of chosen values. PF is hypothesized to be an important aspect of healthy psychological functioning. Despite its potential importance, the distinctness of PF from other constructs has not been adequately demonstrated, and psychometric evaluations of measures designed to assess it are limited. This study aimed at extending current knowledge about PF by examining the construct in 2 help-seeking samples, including panic disorder with agoraphobia (n = 368), clinically relevant social phobia (n = 209), and 2 nonclinical samples including students (n = 495) and individuals visiting an employment office (n = 95). Results across all samples indicate that PF, as measured by the Acceptance and Action Questionnaire (2nd version; AAQ-II), is a unitary construct with a 1 factor model. PF correlated with other variables largely consistent with predictions, differentiated patients from healthy controls, and showed preliminary indications of treatment sensitivity. Incremental validity was partially demonstrated, especially for indices of functioning. Surprisingly, PF also explained unique variance above more established measures for some indices of symptomatology. Results suggest that PF adds some incremental clinical validity, yet further and more stringent tests are required to fully elucidate its strengths and limitations.