Previous studies have substantiated the pivotal role of emotions and teachers in student learning, but the underlying mechanism of teachers and emotions on academic engagement remains underexplored. The present study examined the mediating role of teacher support in the relationship between foreign language learning anxiety (FLLA) and academic engagement in online classrooms. Participants were 415 Chinese university students who completed an online questionnaire. The findings of structural equation modelling indicated that teacher support had a partial mediation effect between FLLA and academic engagement. FLLA exhibited a significant negative impact on academic engagement, both directly and indirectly via perceived teacher support. Through both direct and indirect effects, FLLA could explain 44.8% of the variation in academic engagement and the mediation effect accounted for 47.3% of the total effect. The practical implication is that foreign language teachers should extend more support for learners during online learning, and provide learners with feedback adapted to their emotions.