Online nonprofit communities have increasingly become crucial platforms for fostering customer engagement, providing support, and encouraging collective action. In the context of digital environments, customer engagement—comprising cognitive, emotional, and activating elements—plays a pivotal role in producing several outcomes, including willingness to donate. While prior studies have explored customer engagement in online brand communities, research in the nonprofit sector—where engaged communities willingly contribute their time, finances, and effort toward a shared mission—is still scant. This study applies Social Identity Theory (SIT) to investigate how customer engagement and sense of belonging impact willingness to donate in online nonprofit communities. Data were primarily collected through an online survey questionnaire targeting customers in nonprofit organizations communities. Results indicate that customer engagement significantly influences willingness to donate in this context. Moreover, the study identifies that sense of belonging also plays a critical role in enhancing willingness to donate among participants.