Framed in an interpretative phenomenological approach, this study explored the role of teaching subject (i.e. discipline) in Iranian teachers' online identity construction during the COVID-19 pandemic. Data were collected from teachers of hard and soft sciences through semi-structured interviews, reported practices and online interactions. Data analyses showed that the teachers' online identities involved similarities and differences across their agency, emotion and reflexivity in relation to teaching subject. The authors found that teaching subject influences the relational nature of teachers' identity construction, yet it mediates more profoundly the teachers' agency and practices in online teaching. Moreover, the teachers' emotion and reflexivity were largely constructed in light of their personalised and collegial understandings relevant to sociocultural idiosyncrasies of online education. The findings provide novel implications for characterising teachers' online identities, which help teacher educators approach online teacher education in light of situated understandings that account for teacher identities in greater depth.