Engaging students in writing is a challenge for language teachers. Nowadays, some students have low engagement in writing classes because the genres of writing inside the classes are disconnected from students' real lives. Since today's students are close to social media, integrating it into writing activities can promote student engagement in the learning process. One of the popular social media that can be incorporated into the writing process is "Instagram," which is featured with a photo caption where students can write text messages. The present study aimed to investigate how the implementation of "Instagram" can promote student engagement in EFL writing. This exploratory study used a qualitative research design. The participants of this study were forty-five students enrolled in writing courses and one lecturer of writing. The data were obtained through a semi-structured interview with the lecturer and seven students, classroom observations, and "Instagram" documentation. The findings showed that "Instagram" promoted student engagement in five ways, namely allowing the students to be more actively involved in the learning process, providing a new learning environment for the students, providing greater target readers, allowing the collaboration and interaction between the students, and facilitating the students to choose their own learning style during the process of writing.