The existing literature on collaborative writing (CW) has implied the benefits of CW in improving L2 writing skills and enhancing L2 learners’ writing confidence. However, inconsistent findings exist and few studies have systematically analysed L2 writers’ regulated learning and self-efficacy in the CW context. To that end, this study investigated the impact of peer collaborative writing on twenty-four EFL learners’ development of writing skills and explored their regulated learning and self-efficacy beliefs for CW and L2 writing in a semester-long English course. Their individual writings before and after CW, task discussions, reflective notes, and follow-up interviews were collected and analysed. Findings revealed that learners developed their writing skills after the CW intervention. They performed higher frequencies of co-regulation, self-regulation, and socially shared regulation monitoring episodes in the second and third CW tasks compared to the first task discussion. Analysis of their self-reports also indicated their awareness of self-efficacy and self-regulation for L2 writing. The findings shed light on teaching L2 writing, in that teachers might consider implementing collaborative writing activities to develop learners’ writing skills and encourage them to regulate their own writing process.