An 18-item Portuguese-language version of the CompACT scale has recently been proposed for the Portuguese population. This study aims at conducting the first Confirmatory Factor Analysis of the Portuguese CompACT in participants from two different samples (community adults and women in the post-partum period; total N = 1090). Given that the CompACT had yet to be subjected to gender invariance testing, the present study also presents an invariance analysis between male and female community participants. The measurement invariance of the Portuguese CompACT between community and post-partum women was also examined. The current study additionally explored the scale’s relationships with theoretically relevant outcomes associated in the literature with psychological flexibility. Results showed that the three-factor correlated model of the Portuguese CompACT was an adequate fit for the data taken from the complete sample, with most items presenting statistically and practically significant loading values. The Portuguese CompACT presented acceptable to good internal consistencies for all factors - Openness to Experience (OE), Behavioral Awareness (BA), and Valued Action (VA). Full measurement invariance was found, with results further indicating that community women presented lower scores in BA than community men, and that women in the post-partum period scored higher than community women in BA and VA, and lower in OE. The three subscales of the Portuguese CompACT, that underpin the overarching construct of psychological flexibility, demonstrated different patterns of association from one another with various aspects of individual functioning. Although those associations were in the expected directions with flexibility being associated with lower distress, some forms of psychological flexibility assessed by the CompACT were not significantly associated with measures of positive affect or resilience. This result underscores the difficulty of measuring psychological flexibility as a single construct, as it comprises a number of sub-component processes. Further implications of findings are discussed.