Tourism sustainability has been extensively investigated in prior research. Yet, its relationships with social engagement remain unclear. The objective of this research is to investigate the direct and indirect effects of tourism sustainability and nature affinity on social engagement propensity. It relies on structural equation modeling and on three different studies across different time periods (N = 1405) conducted on tourists from different geographical areas (France and Quebec). The results show that tourism sustainability and nature affinity positively and consistently affect social engagement propensity across different time periods and in both countries investigated. However, the mediators have different effects on the relationships investigated. Nature conservation attitude mediates the effect of tourism sustainability on social engagement propensity for Quebecer tourists, but not for their French counterparts. In contrast, personal tourist experience mediates the effects of tourism sustainability and nature affinity on social engagement propensity for French respondents, but not for those from Quebec. These findings have several theoretical implications for sustainability and nature conservation research and practical orientations for the tourism industry and public policy makers.