Although our society continuously evolves, numerous social and environmental issues remain to be resolved. To address such issues, nonprofit organizations have emerged, and professional sport entities are no exception. By hosting a variety of community outreach programs and/or raising funds through their own charitable foundations, sport entities have become highly engaged with social and environmental issues not only for their local communities but also for the public good. However, given the increasing number of nonprofit organizations in professional sport and the limited amount of financial resources, a critical question for such organizations is how to maintain current donor involvement and eventually guarantee future contributions from potential donors. Even so, researchers have provided limited information on what factors motivate people to make donations specific to the nonprofit organizations that are connected to professional sport teams and athletes. Therefore, this study explored the underlying motivational factors that channel donor behavior by developing an instrument for application in the professional sport setting. In addition, the current study sought to examine the influence of relationship quality on donor behavior in accordance with the impact of donor motivation. Through the instrument development process, three distinctive professional sport donor motivation factors emerged (i.e., fan identification, community support, and charity image) with the measurement showing good psychometric properties in regard to reliability and validity. The results revealed that three motivational factors positively influence donor behavior while the overall measure of professional sport donor motivation did not predict donor behavior well. The results also indicated that both the relationship quality of nonprofit staff members and that of professional athletes had significant effects on donor behavioral loyalty (i.e., future donation intention and positive word-of-mouth) while the influence of nonprofit staff members relationship quality was slightly stronger than athletes relationship quality. The findings of this study generally support the hypotheses and extend the current donor behavior literature. The discussion offers implications for nonprofit managers within the sport industry and academicians pursuing research in the sport philanthropy area, and future research recommendations are suggested.