This paper reports English language teachers' beliefs and perceptions on sustainability concepts as Sustainable Development (SD), Sustainable Development Goals (SDGs), and Education for Sustainable Development (ESD) using both quantitative and qualitative approaches. The data were collected via a questionnaire and semi-structured interview developed by the researcher. The questionnaire was sent to randomly selected teachers, and they responded to the questions online. Likewise, the interview sessions were conducted with volunteered teachers using platforms providing online interaction. The findings showed that the majority of the teachers hold particular perception on environmental aspects but did not consider societal and economic matters to be aspects of SD. Furthermore, the most perceived SDGs are quality of education, no poverty and zero hunger. Most of the teachers rarely had training on ESD during their formal education. The findings also depicted that the participant teachers were of the opinion that SD and ESD were important issues and ESD should be included in the curricula, and developing knowledge and skills on EDS would develop their ability to integrate ESD into their teaching.