Despite the fact that interaction with others (learners and instructors) impacts online learning engagement (OLE), it remains unclear how and when interaction relates to OLE. By integrating social-cognitive theory with ecological systems theory, we theorise a moderated mediation model that links interaction to OLE by exploring the effects of self-regulated learning and family function. Data were collected from 606 undergraduates in China. Our results revealed that interaction positively affected students' OLE. Especially, learner - learner interaction explained a larger proportion of variance than learner - instructor interaction. In addition, the interaction contributed to OLE through the beneficial effects of self-regulated learning. Furthermore, the mediating effect of self-regulated learning on the association between learner - learner interaction and emotional engagement was moderated by family function. The implications for theory, practice, and future research have been discussed.