Considering the importance of sustainable development (SD) in developing countries and lack of research on this topic among Iranian English teachers, this study aims at investigating the Iranian English teachers' awareness and understanding of this concept and the effects of demographic features on their understanding. To this end, a researcher-made 48-item validated Questionnaire that consisted of eight scales was administered among 233 English teachers teaching at high schools, private language institutes, and universities. The data were analyzed through content analysis of the open-ended question and quantitative analysis of the Likert-scale items. One-way ANOVA was used to find out about the effects of gender, workplace, teaching experience, and academic degree on teachers' understanding and awareness. The results revealed that although some teachers could not define the term, or provided a general or wrong definition of the term, more than half of the teachers rightly emphasized the economic aspect, protecting natural resources, and caring for present and future lives of people. It was also found that Iranian English teachers highly supported equity, appreciated and protected the nature, enjoyed diversity, asked for education for sustainable development, and led a frugal life. Finally, it was revealed that none of the demographic features influenced teachers' awareness of SD, except for academic degree and workplace factor that affected three out of eight scales measuring SD. The study ends with several implications for materials developers, English teachers, and syllabus designers.