By applying the broaden-and-build theory of positive emotions to the context of volunteering, we investigated how gratitude is associated with individual (subjective well-being) and organizational (volunteer retention) outcomes through the positive thought–action repertoires of prosocial motivation and volunteer engagement. The data of 924 Chinese volunteers (mean age = 35.7 ± 9.1 years, 26.2% males) from voluntary organizations were collected through an online survey. The indirect effects of gratitude on subjective well-being and retention were analyzed through structural equation modeling performed using Mplus. Gratitude was revealed to be associated with subjective well-being and retention through indirect pathways involving (a) prosocial motivation, (b) volunteer engagement, and (c) a sequential pathway from prosocial motivation to volunteer engagement. Our findings indicate that grateful volunteers tend to exhibit increased intention to help others and actively devote themselves to volunteering, which can contribute to personal and organizational resources.