Summary: This review aims to systematically identify and synthesize qualitative data on adolescents' experiences of the barriers to and facilitators of physical activity to understand whether these differ by socioeconomic position. Multiple databases (MEDLINE, Web of Science Core Collection, PsycINFO, and ERIC) were searched in August 2020. Duplicate title/abstract and full text screening was conducted. Studies were included if they reported qualitative data collected from adolescents (aged 10–19), a measure of socioeconomic position and focused on physical activity. Studies not published in English or published before 2000 were excluded. Relevant data were extracted and methodological quality assessed (in duplicate). Data were analyzed using Thomas and Harden's methods for the thematic synthesis. Four analytical themes emerged from the 25 included studies: (1) social support, (2) accessibility and the environment, (3) other behaviors and health, and (4) gendered experiences. These themes appeared across socioeconomic groups; however, their narratives varied significantly. For example, provision and access to local facilities was discussed as a facilitator to middle and high socioeconomic adolescents, but was a barrier to low socioeconomic adolescents. These findings can be used to inform how different socioeconomic groups may benefit from, or be disadvantaged by, current interventions.