Vocabulary acquisition and retention remain major challenges for lower-secondary EFL learners in Vietnam, particularly for Grade 6 students who are transitioning to a more demanding English curriculum. This study investigated the effects of mind mapping on Grade 6 students’ vocabulary retention and explored their perceptions of using mind mapping in vocabulary learning at Tam Binh Secondary School. A mixed-methods research design was employed with two intact Grade 6 classes: an experimental group receiving vocabulary instruction through mind mapping and a control group taught using traditional techniques. Quantitative data were collected through pre-tests, immediate post-tests, and delayed post-tests, while qualitative data were gathered from student questionnaires, interviews, and classroom observations. The results showed that the experimental group achieved significantly higher scores than the control group in both immediate and delayed post-tests, indicating that mind mapping improved short-term vocabulary learning and long-term retention. Qualitative findings also revealed that most students perceived mind mapping as engaging and helpful for organizing and remembering vocabulary. Classroom observations further showed increased student participation and collaborative learning during mind mapping activities. In general, the study suggests that mind mapping is an effective instructional strategy for enhancing vocabulary retention and learner engagement among Grade 6 EFL learners.