Objectives The link between social support and physical activity has primarily been examined cross-sectionally, with a focus on the direct association between the two variables. In a distinct body of work, there has been growing interest in the role of social support in reducing pain (emotional and physical). We examined the relationship between social support and physical activity over time. We further examined whether reduced pain mediates the relationship between social support and physical activity. Design Data were drawn from Waves 15, 16, and 17 of the Household Income and Labour Dynamics in Australia survey. Methods Mediation models were used to test our hypotheses in (a) a representative sample of 12,517 people residing in Australia, and (b) a subsample of 927 people with a condition that causes chronic or recurring pain. Results Social support was a weak predictor of subsequent physical activity in both the full sample and the subsample of people with a condition that causes chronic or recurring pain. However, in both samples, mediation analyses demonstrated a significant indirect effect of social support on physical activity through reduced pain. Conclusions One pathway through which social support impacts physical activity is by reducing peoples' pain. Increasing and strengthening peoples' social support networks may confer benefits for their physical activity levels, including among those whose physical activity is limited by pain. Statement of contribution What is already known on this subject? Social support can have a positive effect on health behaviours, including physical activity. There is somewhat inconsistent evidence for a positive direct relationship between social support and physical activity. Pain can be a barrier to physical activity, but may be attenuated by social support. What does this study add? Social support affected physical activity indirectly by reducing peoples' pain. This was true for both the general population and a subsample with a chronic pain condition. Improving peoples' social support networks may confer benefits for their physical activity.