Driven by the desire to understand classroom practices that promote L2 student engagement, this study examined the psychological mechanisms by which two autonomy-supportive teaching behaviours, namely provision of choice and task relevance, exerted their influence on different components of L2 engagement. The development-in-sociocultural context model was used as the guiding theoretical framework while structural equation modelling was employed as the main data analysis technique. Analysis of self-report data from 413 EFL students in Vietnam suggested that task relevance was a more salient predictor of L2 engagement than provision of choice both directly and indirectly. Furthermore, students’ perceived usefulness of L2 learning and their emotional reactions to L2 tasks play central roles in channelling the effects of provision of choice and task relevance on L2 engagement. The study findings were discussed in light of existing evidence from different subfields of second language acquisition including learner psychology and instructed second language acquisition. Implications for practice were also suggested to inform L2 task design and delivery.