The study aims to explore a conceptual model for the sustainable reciprocity relationships in sport-based initiatives and empirically test the model and its underlying mechanism in the context of a real sport-based national initiative. Adapting a seminal work from social work literature as a theoretical framework and the following measurement for the social impact of sport from sport management literature, a conceptual model addressing sport participation, gratitude, social benefits, and prosocial behaviors is presented. Two separate surveys were conducted through face-to-face interviews with independent and random samples representing the Singaporean residents in October 2014 for Study 1 (n = 500) and February 2015 for Study 2 (n = 501). The results demonstrate that the frequency of participation in a range of daily sport activities of a national sport initiative positively influences the perceived value of social capital and health literacy through the mediation effect of gratitude. The study also demonstrates that participation in sport activities positively influences prosocial behavioral intention through the serial mediation effect of gratitude and social capital. The findings implies how we better understand and utilize the dynamic power of gratitude to sustain the win–win relationships to multi-stakeholders in the contexts of sport-based initiatives based on the nature of reciprocity.