<p>Learners’ engagement has been identified as a crucial factor contributing to effective teaching and learning (Chiu, 2022); yet research focusing on this particular topic remains limited (Werang & Leba, 2022). In the time of the COVID-19 pandemic, in-depth investigations into this aspect have become increasingly urgent, especially in such contexts as Vietnam where online instruction has become “a new normal”. This descriptive case study, as a part of a more comprehensive study, responded to such as pressing call by exploring two aspects: (1) adolescent EFL learners’ engagement levels, and (2) factors influencing learners’ engagement in synchronous online classes. Quantitative data were collected through questionnaires administered to 193 Vietnamese adolescent EFL learners at an EFL language center in the Mekong Delta, Vietnam. Results revealed that learners’ perception of adolescent learners’ engagement level was high. Specifically, learners were more behaviorally engaged compared to the other dimensions, namely emotional, cognitive and agentic engagement. Two categories of factors, namely teacher-related aspects and teaching content and activities were found to have the most significant influence on learners’ engagement. These findings provide teachers, curriculum developers, and institution administrators with important practical implications for measures to effectively foster higher engagement in virtual classrooms.</p><p> </p><p><strong> Article visualizations:</strong></p><p><img src="/-counters-/edu_01/0555/a.php" alt="Hit counter" /></p>