Psychological Flexibility is an essential concept in Contextual Behavioural Sciences. The development of instruments is vital for its study, and it is an opportunity to reflect on this concept. Among the measures of this construct, Psy-Flex shows promise due to comprehensiveness in assessing the six facets of psychological flexibility and its pragmatic value. In the present study, we sought to translate Psy-Flex to Portuguese and adapt it transculturally for use in Portugal and Brazil. The translation was done independently by Brazilian and Portuguese researchers, and a consensus procedure was done to identify a synthesis. A total of 873 adults from Portugal and Brazil were involved in this study. The participants completed several questionnaires with ACT-related measures (e.g., cognitive fusion, mindful attention) and instruments measuring conceptually related variables (e.g., positive mental health). The results show good psychometric properties of PsyFlex. The one-factor structure of the original instrument was confirmed in both the Portuguese and Brazilian samples. Confirmatory factor analysis revealed a good fit of the model to the data (CFI = 0.985, RMSEA = 0.055) of both countries. Furthermore, the Psy-Flex showed convergent validity with related measures in a conceptually expected way. As a transcultural instrument, we argue that both the similarities and differences across samples suggest the broad human nature of psychological flexibility while retaining its context sensitivity.