Perfectionism is related to precompetition emotions in athletes. However, it is unclear why this is the case. In the present study, the authors sought to determine whether perfectionistic cognitions explain this relationship and mediate the relationships between self-oriented perfectionism (SOP), socially prescribed perfectionism (SPP), and general precompetition emotions and multidimensional anxiety and anger. The authors adopted a three-wave longitudinal design and examined between- and within-person effects in a sample of 352 youth footballers (M age = 14.03 years, SD = 2.30). At the between-person level, perfectionistic cognitions mediated the relationships between SOP, SPP, and all general precompetition emotions plus multidimensional anxiety and anger. At the within-person level, perfectionistic cognitions mediated the relationships between SOP, SPP, and general anxiety and anger plus multidimensional anxiety and anger. Our findings imply that athletes higher in SOP and SPP experience more anxiety and anger when the frequency of perfectionistic cognitions increases in the lead-up to competition.