Before second language writing (SLW) teachers' digital practices can be supported, their needs must first be understood. To accomplish effective technology-enhanced instruction, SLW teachers must blend their knowledge of composition theory, second language acquisition, and multimodal composition technologies. However, many teachers struggle to do this, which highlights the need for research addressing the cognitive aspects that influence digital instruction. This case study reports on an investigation of three in-service university SLW teachers' Technological Pedagogical Content Knowledge (TPACK) during a digital reflective portfolio module. Data from an online survey, instructional content, classroom observations, and semistructured interviews were triangulated to uncover the nature of SLW teachers' TPACK, including which factors supported and constrained their use of technology. Findings suggest that pedagogical content knowledge played a dominant role in how the teachers used technology. The teachers' TPACK was enhanced by professional beliefs about the importance of multimodality and contextual factors involving institutional support and communities of practice. However, it was constrained by limited self-efficacy and pedagogical beliefs concerning the influence of technology on student learning and student engagement. This study contributes to a growing body of research on how to support language teachers in their digitally mediated practices.