Theme-based instruction (TBI) is deemed one of the innovative language teaching approaches. However, re-search on TBI, particularly teachers’ viewpoints on TBI in English as a foreign language education at high schools in Vietnam, is still scarce. The current study aims to explore EFL teachers’ perceptions of the imple-mentation of TBI at high schools in Dalat City, Vietnam. The explanatory sequential mixed-methods approach was employed with sixty-five Vietnamese teachers of English. The quantitative data obtained from the question-naire were statistically processed (i.e., mean and standard deviation), and the qualitative data gathered from the semi-structured interviews were analyzed through content analysis. The findings generally indicated that the participants positively perceived the TBI implementation at high schools in the capital city of Lam Dong Prov-ince, Vietnam. Specifically, TBI provided EFL learners with merits, e.g., increasing opportunities for improving English vocabulary and competency, strengthening cooperation, enhancing problem-solving and presentation skills, developing cognition, and connecting content to the real world based on their own needs and interests. Additionally, students were expected to take responsibility for their learning, while teachers assumed the roles of resources, facilitators, and motivators. Strikingly, the research revealed discrepancies in perceptions of Task-Based Instruction (TBI) implementation between high school EFL teachers who received formal training from the Department of Education and Training of Lam Dong Province and those who had not undergone such training.