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As interest in sluggish cognitive tempo (SCT) increases, a primary limitation for the field is the lack of a unified set of symptoms for assessing SCT. No existing SCT measure includes all items identified in a recent meta-analysis as optimal for distinguishing between SCT and attention-deficit/hyperactivity disorder (ADHD) inattention. This study evaluates a new self-report measure for assessing SCT in adulthood, the Adult Concentration Inventory (ACI), which was developed in response to the meta-analytic findings for assessing SCT. Using a large, multiuniversity sample (N = 3,172), we evaluated the convergent and discriminant validity and reliability of the ACI. We also evaluated the ACI measure of SCT in relation to self-reported demographic characteristics, daily life executive functioning, socioemotional adjustment (i.e., anxiety/depression, loneliness, emotion dysregulation, self-esteem), and functional impairment. Exploratory confirmatory factor analyses resulted in 10 ACI items demonstrating strong convergent and discriminant validity from both anxiety/depressive and ADHD inattentive symptom dimensions. SCT was moderately to-strongly correlated with daily life EF deficits, poorer socioemotional adjustment, and greater global functional impairment. Moreover, SCT remained uniquely associated in structural regression analyses with most of these external criterion domains above and beyond ADHD. Finally, when internalizing symptoms were also covaried, SCT, but not ADHD inattention, remained significantly associated with poorer socioemotional adjustment. These findings support the use of the ACI in future studies examining SCT in adulthood and make a major contribution in moving the field toward a unified set of SCT items that can be used across studies.