The Covid-19 outbreak posed a compelling challenge for teacher education programs regarding preparing pre-service teachers for online language teaching. In this study, nine Turkish EFL pre-service teachers were involved in a six-week online teaching project, which provided them with theoretical and practical training on online language teaching. They were engaged in lesson planning for one synchronous and two asynchronous lessons and taught these lessons to EFL students. This case study explored pre-service teachers’ technological pedagogical content (TPACK) development via reflection reports, an online semi-structured interview, lesson plans, recordings of online lessons and posts on the asynchronous platform. The data analysis was conducted through thematic analysis. The findings demonstrated that participation in the online teaching project facilitated the participants’ overall TPACK development. However, some pre-service teachers encountered challenges in effectively integrating technology with content and pedagogy. Specifically, lacking contextual knowledge hindered their ability to design and conduct online lessons successfully. The study underscores the importance of adopting a situated learning-by-doing approach to TPACK development, the value of pedagogical mentoring provided by teacher educators and field experience for pre-service teachers to gain competence across all subcomponents of TPACK.