Adult separation anxiety disorder (ASAD) is characterized by developmentally inappropriate and excessive fear or anxiety concerning separation from those to whom the individual is attached. Despite the high rates of this diagnosis among Portuguese adults, there is a lack of measures to assess it. In this study, we assessed the psychometric properties of a Portuguese adaptation of the Adult Separation Anxiety questionnaire (ASA-27) on a sample of 267 adults (72.7% women) aged 18-80 years (M = 40.5, SD = 13.1). Factor structure, internal consistency, and convergence validity were examined. This study confirmed the single-factor structure of the Portuguese version of ASA-27. Consistency was high for the total sample (ω = .92) and by gender (ω = .93 and 92, men and women groups, respectively). The scale was positively related to the Portuguese version of State-Trait Anxiety Inventory (STAI) (r = .57, p< .001, for both State and trait anxiety scales) and Composite Codependency Scale total score (r = .29, p< .001). In addition, the ASA-27 total score showed incremental validity in the explanation of anxiety measured by STAI. In conclusion, results show that the Portuguese version of the ASA-27 is a reliable and valid measure of ASAD.