The significance of environmental education has become more evident, driven by global concerns surrounding climate change. In Vietnam, a country facing significant environmental challenges, developing environmental knowledge of elementary school teachers is crucial. This study aimed to validate a questionnaire for assessing professional knowledge of elementary school teachers in teaching environmental issues, originally designed in the Korean context. Recognizing the need for adaptation to the Vietnamese educational system, language, and local environmental issues, the initial 39-item questionnaire underwent a validation process. A total of 86 elementary school teachers participated in the construct validity and reliability, while eight qualified professionals evaluated the content validity. The results indicated that content validity indices for each item ranged from 0.88 to 1.00, and a scale-content validity index was 0.90. Exploratory factor analysis confirmed construct validity, organizing the 27 items into seven dimensions. The Cronbach's [alpha] coefficient was 0.86, which confirmed the questionnaire demonstrated a high level of reliability. Therefore, it is a reliable instrument for assessing elementary school teachers' professional knowledge in teaching environmental issues, which is relevant in the Vietnamese context. The validated instrument supports the development of professional development programs and contributes to the broader goal of enhancing environmental education in Vietnam.