Background: In basketball, children's main reason for playing is to have fun, whereas outcomes such as skill acquisition and long-term development are often perceived by adults as the main motive for children's engagement in sport. However, no studies have yet examined whether the game outcome could influence motivational variables longitudinally over the course of a season. The novelty of the present study resides in the longitudinal exploration, the multilevel approach, and the analysis of motivational climate as a mediator variable between game outcome and enjoyment, perceived competence, and intention to be physically active. Purpose: The purpose of this study was to analyze whether under-12 basketball players' who won versus lost games had different perceptions of their enjoyment, perceived competence, and intention to be physically active throughout a season, considering motivational climate as a mediator. Method: The participants were 94 boys and two girls from eight basketball clubs (M[subscript age] = 9.72 years, SD = 1.70 years, age range = 9-11 years). The study followed a six months longitudinal design. The independent variable was the game outcome (win-loss). The dependent variables were enjoyment, perceived competence, and intention to be physically active. Motivational climate was a mediator variable. Data were collected at the end of each of the 56 games throughout 14 game days over a basketball formal season. Longitudinal multilevel regression analysis was conducted within player-level and between player-level with the repeated measures representing the game-to-game variability. Results: We found that at the within player level, motivational climate mediated the effect of game outcome as follows. On game days, when the game is won, players perceive the climate as more ego-related, which in turn reduced enjoyment and increased perceived competence. On game days, when the game is lost, players perceive the climate as more task-related, which in turn increased enjoyment, perceived competence, and intention to be physically active. Conclusion: Game outcomes did not directly affect players' feelings, but motivational climate mediated the effect between game outcomes and motivational elements within players throughout a season. This study shows that the competitive nature of sport is not a deterrent to youth positive experiences provided adults minimize the emphasis of game outcomes over personal factors such as competence and enjoyment, through motivational climate.