Understanding the potential transformation brought about by integrating the flipped classroom methodology with rich and dynamic learning platforms like the Metaverse, in terms of usability, students' academic performance, and intrinsic motivation, holds critical importance for adopting innovative strategies in the field of education. This study describes an attempt to integrate flipped learning method into a medical English course by using an active learning strategy supported by a Metaverse-based environment. Therefore, the present study aims to compare the synchronous online flipped medical English classroom supported by a Metaverse-based platform and the conventional medical English classroom supported by the synchronous distance education and to examine the effectiveness of Metaverse-powered synchronous online flipped learning on medical students' academic achievement. For this purpose, a mixed method was used to conduct the study. A quasi-experimental design was adopted for the quantitative dimension of the research. The students in the experimental group engaged in the activities with the help of the Spatial AR environment, a Metaverse platform enhanced with three-dimensional (3D) objects and allowing peer interaction, In the qualitative aspect of the study, the data were collected through a focus group interview, in which a semi-structured interview form was used. The participants of the study consisted of 100 first-year medical students at a state university in Türkiye. The results revealed that the experimental group students outperformed the control group students regarding academic achievement. In addition, the mean score of male students was higher than that of female students in the experimental group. As a result of the qualitative data analysis, eight themes emerged from the created codes. Students stated that learning medical English through the flipped classroom supported by Metaverse-based technology was enjoyable and that it increased their interest. Regarding learning outcomes, the most prominent benefits were permanent learning, comprehending the subject easily, deep learning, and embodying abstract concepts. Concerning language skills, students expressed that synchronous online flipped learning assisted by Metaverse-based technology helped develop their reading and listening skills as well as to learn new vocabulary. On the other hand, some students reported negative thoughts about the Metaverse platform due to technical problems and about flipped classroom activities because they found some of them time-consuming. The present study suggested that synchronous online flipped medical teaching model assisted by the Metaverse could positively affect the academic achievement of students. However, it is recommended to investigate the most effective practices that will meet the needs of students in different contexts.