Though positive social norms can promote desired health behaviours, findings surrounding injunctive norms (i.e., perceptions of what others think one should do) in relation to physical activity are equivocal. One potential explanation pertains to the role of autonomy support and autonomy satisfaction, which are also positive correlates of physical activity engagement. That is, injunctive norms from significant others may undermine one's autonomy satisfaction, particularly if the significant others are less autonomy-supportive ("Others think I should be active, so I feel pressured"). The current study examined whether injunctive norms negatively predicted individuals' autonomy satisfaction, and whether this relationship was moderated by their perceptions of autonomy support. Responses from 497 participants at baseline and 351 participants at a two-week follow-up (Mage = 20.5 ± 4.7 years) were analyzed using structural equation modeling. Consistent with previous research, higher autonomy satisfaction related to more intrinsic forms of behavioural regulation, which in turn translated to greater physical activity at time two. As expected, injunctive norms from significant others negatively related to autonomy satisfaction after controlling for perceptions of autonomy support, b = -.08, p = .007. Though the interaction term was nonsignificant (p = .235), patterns of moderation were observed, where the negative relationship between injunctive norms and autonomy satisfaction was exacerbated at lower levels of autonomy support, b = -.11, p = .002, compared to higher levels, b = -.01, p = .667. These results highlight the potential downside of injunctive norms for autonomous physical activity engagement.