The purpose of this study is to shed light on the mediating effect of heutagogical activities on the relationships between (1) social learning environment & self-determined learning and (2) students’ psychological needs & self-determined learning. The proposed model hypothesised that self-determined learning is dependent on the social learning environment, student psychological needs, and heutagogical activities. This study fills the knowledge gap by applying the partial least squares structural equation modelling (PLS-SEM) to test the proposed model based on a sample of 626 postgraduate students from a higher education institution in Malaysia. The findings revealed that heutagogical activities had mediating effect on postgraduates’ self-determined learning. These are new insights on self-determined learning as no studies have examined the mediating effect of heutagogical activities through structural equation modelling analysis. Overall, this study offers valuable contributions to the empirical evidence and body of literature on heutagogy by providing an extended understanding of the mediating role of heutagogical activities in postgraduates’ self-determined learning. The findings suggest that more efforts are needed to refine the learning activities in postgraduate education for the postgraduates’ development of self-determined learning. The theoretical and practical implications, limitations, and suggestions for future research are also discussed.