In Western countries, the popularity of running events has increased exponentially during the last three decades. However, little is known about the profile of non-participants. This knowledge is crucial to tailor promotional actions towards people who are currently not participating. Therefore, this study aimed: (1) to examine which socio-ecological factors are related to participation in running events, (2) to give an overview of the barriers towards participation, and (3) to examine differences in barriers depending on gender, age and educational level. Flemish adults (n = 308) completed an online questionnaire about their socio-demographic and psychosocial characteristics, physical activity levels, participation in running events and barriers towards participation. Results showed that motivation, family social support, knowledge about running events and physical activity levels were associated with participation in running events. Among non-participants, the main barriers were bad physical condition, lack of time and lack of interest. In participants, lack of time, distance to the event and financial barriers were most prevalent. With some exceptions, barriers were relatively similar across socio-demographic subgroups. This study confirmed a democratization among participants of running events and provided evidence about which barriers should be tackled to increase participation among population subgroups that are currently underrepresented in such events.