Purpose: This study makes an enquiry of the existing sustainable development goals (SDGs) knowledge of English language teacher trainers (ELTTs), who remain an overlooked stakeholder in education for sustainable development. Despite the literature on SDG integration into traditional teacher education curriculum, how massive open online courses (MOOCs) can help with capacity building of ELTTs is unclear. This study aims to further explore how the knowledge and capacities of ELTTs on SDGs could be promoted through MOOCs. Design/methodology/approach: For the study's action research approach, data of 28 ELTTs from higher education institutions in Ankara, Türkiye, were collected for six weeks using a survey, reflective reports and focus group interviews. Findings: ELTTs with no previous training experience have a partially fragmented understanding of SDGs. MOOCs proved to be highly practical and cost-effective in building ELTTs' capacity for building knowledge of and improving motivation to address SDGs in training programs. Further findings are drawn from ELTTs' reflections on the lack of pedagogical content knowledge and collegial interactions in MOOCs. Originality/value: There is little to no literature on ELTTs' existing knowledge of SDGs and whether MOOCs can be used for their capacity building to achieve SDGs. The present findings may encourage higher education institutions to launch professional development programs that will equip ELTTs with the competencies to apply education for sustainable development in in-service English language teaching and teacher training programs.