This study aimed to examine the factor structure, the simplex structure, and the self-determination continuum of the Youth Behavioral Regulation in Sport Questionnaire (YBRSQ); to test longitudinal invariance of the YBRSQ and to examine differential item functioning in the YBRSQ responses as a function of sex, type of sport, and competitive level; and to explore the dynamics of change and stability of motivational regulation across the competitive season in a sample of 736 adolescent athletes involved in intensive training settings across 3 measurement points (beginning, middle, and end). Results provided evidence of a simplex structure of YBRSQ scores and revealed differences between self-determination-theory-based measures of motivation in various contexts. Results revealed partial strict temporal invariance of the YBRSQ and did not reveal differential item functioning. Finally, the results demonstrated an increase in amotivation and external regulation and a decrease in intrinsic motivation across the season, probably because of daily pressures.