This prospective longitudinal study evaluated changes in psychological distress among adolescents, pre-pandemic to intra-pandemic, the extent to which within-person and between-person differences in trait multidimensional perfectionism were associated with such changes, and the role of stress in explaining associations between perfectionism and psychological distress. Adolescents (N=187; 80% female; 78% White, 7% Asian Canadian, 2% Indigenous Peoples in Canada, 2% Black or African Canadian, 2% Latin Canadian, or 9% Other; Mage =17.96 years) completed online surveys assessing perfectionism (i.e., self-oriented perfectionism and socially prescribed perfectionism), depression, anxiety, and stress pre-pandemic (i.e., March 12, 2020 or earlier) and during Ontario, Canada's first (i.e., March 13, 2020 to July 24, 2020) and second (December 26, 2020 to February 7, 2021) government-mandated lockdowns. Between-person differences and within-person changes in multidimensional trait perfectionism were associated with increases in psychological distress and perceived stress. Perceived stress served as an intermediary pathway linking multidimensional trait perfectionism to psychological distress during the pandemic.