Writing is an essential skill for academic development within any disciplinary area. Despite the rapidly growing body of research on the various aspects of second language writing, research on writing self-efficacy remains scarce. This study investigated the relationship the between writing self-efficacy and writing proficiency in English as a second language. In this cross-sectional study, 304 Malaysian undergraduate students completed a writing self-efficacy questionnaire. The participants’ writing proficiency was assessed using two different writing tasks. The results showed that there was a significant difference in writing self-efficacy among the three writing proficiency groups. It was also found that science students had significantly higher writing self-efficacy than those in social sciences. Limitations of the study and Implications for second language writing instruction are also discussed.