The present study explores the efficiency of the flipped classroom approach on English for Academic Purposes (EAP) students' academic vocabulary acquisition in comparison with the conventional teaching approach. The efficiency is examined by evaluating students' post-test performance, self-perceived mental effort employed in completing post-test tasks, and perceptions regarding the learning experiences. To this end, 60 undergraduates divided into flipped (E) and conventional (C) group participated in the survey. Quantitative data analysis revealed that the flipped classroom approach showed higher instructional efficiency than the conventional approach as the E group significantly outperformed the C group and reported investing considerably lower mental effort in completing the post-test tasks. In addition, the E group expressed significantly more positive perceptions toward the learning experience than the C group. Considering the positive results obtained in the study, the article points out the use of the flipped approach as an example of good practice for enhancing academic vocabulary acquisition in EAP context.