The functional theory of self-determination (fSDT) defines and operationalises self-determination within a human agentic context. It emerged from research on adolescents with disabilities, however has been increasingly applied to youth without disabilities. While comparability has been evaluated in youth with and without disabilities, it has not been explored across cultures. The purpose of this study was to explore the cross-cultural comparability of the fSDT in a sample of Italian and American adolescents. We were specifically interested in examining the universal aspects of the self-determination construct, as well as specific differences in the operationalisation of self-determination across cultures. The findings tentatively suggest that the construct of self-determination is comparable across Italian and American adolescents; however, there are specific differences in the measurement and operationalisation of self-determination across cultures. Directions for future research are discussed.