The number of teaching assistants (TAs) working in mainstream schools has soared in recent years as students with special educational needs (SEN) are integrated into regular classrooms. However, research on TAs is rare. This study investigated whether and how work engagement mediates the relationship between self-efficacy and job satisfaction among 292 Chinese TAs working in Hong Kong mainstream schools. Survey data were analyzed using partial least squares structural equation modeling (PLS-SEM). Supported by the social cognitive theory of self-efficacy and self-determination theory of work engagement, our results showed that TAs' self-efficacy is positively related to their job satisfaction through the mediation of work engagement. Implications for an appropriate focus on enhancing TAs' self-efficacy through classroom experiences and training courses are discussed.