Background Levels of physical activity (PA) decrease when transitioning from adolescence into young adulthood. Evidence suggests that social support and intrapersonal factors (self-efficacy, outcome expectations, PA enjoyment) are associated with PA. The aim of the present study was to explore whether cross-sectional and longitudinal associations of social support from family and friends with leisure-time PA (LTPA) among young women living in disadvantaged areas were mediated by intrapersonal factors (PA enjoyment, outcome expectations, selfefficacy). Methods Survey data were collected from 18-30 year-old women living in disadvantaged suburbs of Victoria, Australia as part of the READI study in 2007-2008 (T0, N = 1197), with follow-up data collected in 2010-2011 (T1, N = 357) and 2012-2013 (T2, N = 271). A series of singlemediator models were tested using baseline (T0) and longitudinal data from all three time points with residual change scores for changes between measurements. Results Cross-sectional analyses showed that social support was associated with LTPA both directly and indirectly, mediated by intrapersonal factors. Each intrapersonal factor explained between 5.9-37.5% of the associations. None of the intrapersonal factors were significant mediators in the longitudinal analyses.Conclusions Results from the cross-sectional analyses suggest that the associations of social support from family and from friends with LTPA are mediated by intrapersonal factors (PA enjoyment, outcome expectations and self-efficacy). However, longitudinal analyses did not confirm these findings.