For several decades, there has been an increase in studies on second language motivation, one of the most salient topics in individual difference research in second language acquisition, guided by theories and methods from related fields. Self-determination theory (SDT) is one of the most influential theories to provide a comprehensive framework for investigating language learning motivation. To date, numerous SDT-related studies have been performed to explore ways to develop more self-determined types of motivation. However, research on the relationship between self-determined types of motivation and other psychological variables has been limited. To address this gap, the present study investigated the complex relationships between autonomous motivation, buoyancy, boredom, and engagement in a sample of 561 Chinese senior high school English as a foreign language (EFL) learners. Data were collected through a composite questionnaire measuring students’ autonomous motivation, buoyancy, boredom, and engagement in EFL learning. Chain mediation analysis was used to test the complex relationships among these variables. The results show that autonomous motivation directly affected student engagement in EFL learning and autonomous motivation also indirectly affected student engagement in EFL learning through the separate mediation of buoyancy and boredom in EFL learning as well as the chain mediation of both mediators. The results support SDT and offer some pedagogical implications for teachers and educators.