This article describes the development and evaluation of the Engaged Living Scale (ELS) as a new self-report, process-specific measure to assess an engaged response style as conceptualized in acceptance and commitment therapy (ACT). The psychometric properties of the ELS test scores were evaluated in both a nonclinical sample (N = 439) and a clinical sample consisting of chronic pain patients who participated in a study on the effects of an online ACT intervention (N = 238). Item analysis and exploratory factor analysis in the nonclinical sample suggested a 16-item version of the ELS with 2 subscales, Valued Living (10 items) and Life Fulfillment (6 items). A bifactor model with 2 specific factors and 1 general underlying factor showed the best fit in confirmatory factor analyses in the chronic pain sample. In both samples, the scores on the ELS and its subscales showed good internal consistency and construct validity by consistent patterns of relationships with theoretically related process and outcome variables, such as psychological well-being, anxiety/depression, acceptance, mindfulness, and pain interference in daily life. Furthermore, in the chronic pain sample, the ELS showed incremental validity in explaining anxiety and depression, positive mental health, and pain interference beyond both acceptance and mindfulness. This study suggests the ELS shows promise as a useful tool for the measurement of an engaged response style, enabling more comprehensive evaluation of working mechanisms of ACT.