Online interaction has been associated with positive outcomes, academically and psychologically. Regarding the latter, social connectedness has been recognised as an important outcome on the basis of its link to subjective well-being and course satisfaction. In adult educational settings, how online interaction can foster social connectedness has not been widely examined. The study investigates how (1) adult learners' online participation operationalised as discussion contribution, collaborative facilitation, and social interaction, and (2) their perceived online interaction quality are related to their feeling of social connectedness. The role of online interaction quality related to the three types of online participation and social connectedness is also examined. A questionnaire was developed to collect data from adult learner participants in Belgium (N = 170). Multiple regressions and mediation analysis were conducted. The result shows that collaborative facilitation and online interaction quality significantly contribute to adult learners' perceptions of social connectedness. Furthermore, the finding reveals that online interaction quality fully mediates the relationship between discussion contribution, social interaction, and social connectedness. Thus, the implications for research and instructional design are discussed.