Background: In South Africa, the expanding epidemic of non-communicable diseases is partly fuelled by high levels of physical inactivity and sedentary behaviour. Women especially are at high risk, and interventions promoting physical activity are urgently needed for girls in their adolescence, as this is the time when many girls adopt unhealthy lifestyles. Objective: This qualitative study aimed to identify and describe facilitating factors and barriers that are associated with physical activity among adolescent girls in rural, north-eastern South Africa and, based on these, to develop a model for promoting leisure-time physical activity within this population. Design: The study was conducted in and around three secondary schools. Six focus group discussions were conducted with adolescent girls from the schools, and seven qualitative interviews were held with sports teachers and youth leaders. The data were subjected to thematic analysis. Results: Seven thematic areas were identified, each of which was associated with the girls’ self-reported levels of physical activity. The thematic areas are 1) poverty, 2) body image ideals, 3) gender, 4) parents and home life, 5) demographic factors, 6) perceived health effects of physical activity, and 7) human and infrastructural resources. More barriers to physical activity were reported than facilitating factors. Conclusions: Analysis of the barriers found in the different themes indicated potential remedial actions that could be taken, and these were synthesised into a model for promoting physical activity among South African adolescent girls in resource-poor environments. The model presents a series of action points, seen both from the ‘supply-side’ perspective (such as the provision of resources and training for the individuals, schools, and organisations which facilitate the activities) and from the ‘demand-side’ perspective (such as the development of empowering messages about body image for teenage girls, and encouraging more parental involvement). The development of physical activity interventions that incorporate this supply- and demand-side model would represent an additional tool for ongoing efforts aimed at tackling the expanding non-communicable disease epidemic in South Africa, and in other resource-constrained settings undergoing rapid health transitions.