Technology-mediated communication has become ubiquitous and digital multimodal learning has been increasingly integrated into second language education. Drawing on the concepts of transmediation and interpretant in social semiotic theories, this study explores the potential of digital storytelling (DST) in promoting EFL learners' willingness to communicate (WTC) through transmediation. A DST workshop was offered to 69 undergraduate students in China, and the workshop was intentionally designed to consist of prescript writing, video making, and postscript writing, which induced transmediation. Data sources included DST videos produced by the participating students, their prescripts and postscripts, and interviews, with a micro-level analysis conducted on two focal students. The results showed that the participants' L2 WTC significantly increased in the writing of the postscripts than the prescripts. Three interpretants generated through transmediation were identified to be associated with students' increased L2 WTC: expanded retrospective narratives, deepened perception, and evoked sentiments. Based on these findings, pedagogical implications for implementing DST projects as a fine example of multimodal composing practices in promoting EFL learners' L2 WTC and multimodal communicative competence are provided.