There are immediate calls to action across all business sectors to address the adverse effects of climate change. The sport industry is not immune to these calls—practitioners have engaged participants to further the environmental initiatives of their events through creative messaging campaigns with mixed success. We examine the differential effects of internal and external constraints on sustainability intentions by market segment of a running event’s participants. This furthers our understanding of how to engage sport participants in sustainable behaviors at a sport event and improves the sustainable behavior outcomes of messaging campaigns. Our results support the principles of constraint theory and that internal constraints need to be addressed before external constraints are relevant. Internal constraints, such as lack of knowledge and lack of worth, predict variance in intentions to act sustainably (i.e., to dispose of waste correctly). After controlling for internal constraints, the external constraints of lack of interest by others to act sustainably, and lack of access and time, explain additional variance in sustainability intentions. We also found that each market segment varies on the impact of constraints that inhibit sustainability intentions, with the amount of variance in sustainability intention explained by such constraints varying from as low as 11.4% in one segment to as high as 33.1% in another. We then provide specific recommendations for future research and discuss how practitioners can use these results to ameliorate these constraints and increase sustainability intentions.