Based on the tenets of self-determination theory, the Sport Motivation Scale-II is an 18-item instrument that consists of six subscales. Despite the numerous studies that have been conducted using self-determination theory in organized youth sport contexts, measures available to assess self-determined forms of motivation across socio-cultural contexts are scarce. This is particularly important because there are socio-cultural contexts that have diverse reward systems, competitive structures, and convey different social norms, values, and cultural nuances. The existence of measures available to assess self-determined forms of motivation across socio-cultural contexts may enable the development of a robust knowledge base that informs research-to-practice partnerships and potential changes across a complex youth sport landscape. The present study aimed to validate the factor structure of the Sport Motivation Scale-II with a sample of 239 Portuguese adolescent athletes. The participants’ ages ranged between 12 and 17 years old ( M = 14.39; SD = 1.35). Confirmatory factor analysis was conducted to achieve the aim of this study. Findings showed that the six-factor model used by the authors presents a good adjustment concerning sports participants’ level of motivation toward sports practice. Moving forward, the Sport Motivation Scale-II may be used in future studies to help provide an understanding about athletes’ self-determination. Several practical and theoretical implications are provided.