Differentiated instruction (DI), an educational philosophy that aims to cater for learner diversity, has been advocated by scholars and teachers over the past two decades. Nevertheless, little is known about how teachers practise DI in online teaching, particularly in the context of English language teaching during the outbreak of the COVID-19 pandemic. To fill this gap, this single participant case study aimed to delineate concrete ways of implementing DI during an online English writing lesson for English as a second language (ESL) learners in a Hong Kong primary school. To provide a thick description, multiple sources of data were extracted from a rich data pool of a two-year longitudinal teacher professional development project which included lesson recordings, post-lesson debriefing meeting recordings, and student work. The findings suggested that the case teacher employed various DI strategies (i.e., providing chances for all student participation, using multisensory pedagogical designs, using information technology) to cater for learner diversity during online English teaching. It is argued that despite the potential challenges, it could be feasible and manageable to employ some DI strategies to address learner diversity during online English teaching. This study contributes to DI theory and practice by deepening our understanding of implementing DI in the context of online English teaching and providing a valuable reference for educational practitioners. It also advances our knowledge of how technology could be leveraged to support DI implementation.