Socially excluded groups are at higher risk of low well-being and poor health. The link between social exclusion and health inequities is complex, and not being involved in society makes it difficult to be reached by standard prevention programs. Sport-for-development (SFD) programs are low-threshold and may be promising settings for inclusive actions. We explore the underlying mechanisms through which SFD might have an impact on social inclusion and examine the necessary conditions that work as a catalyst for these underlying mechanisms.A realist evaluation approach was adopted. A non-profit SFD organization in a middle-large city in Flanders, Belgium, formed the setting for a single case study. Document analysis, participatory observations, interviews, and a focus group, were sources for identifying necessary context elements and essential mechanisms through which SFD could promote its participants' health and wellbeing.Among the most efficient mechanisms triggered by the Foundation's activities are learning by fun, connecting with peers (of whom some serve as role model) and engaging as a volunteer with some responsibilities. Building trust in oneself and in others is a necessary process throughout all these mechanisms. Facilitating context factors include the activities' accessibility and unconditional approach (creating a sense of safety), the popularity of the first division football team the Foundation is associated with (leading to a sense of belonging), a steady network of social partners and a strongly positive relationship with the SFD coach(es).Our findings demonstrate that a SFD setting may be a vehicle for engaging hard-to-reach population groups. It enhances socially vulnerable persons' sense of competence and connectedness, leading to opportunities to improve life and work skills transferrable outside SFD settings. Based on these findings, suggestions are provided that may enhance the field and help to develop feasible (policy-led) interventions designed to promote social inclusion.