This study aimed to determine how teachers' online written corrective feedback (WCF) affected university students' self-regulation in writing and how the effects differed among students of different English proficiency levels. The participants were 27 second-year university students enrolled in a required online writing course. Throughout the 16-week course, the students received online WCF on their writing through Google Docs. Results from pre- and post-questionnaires and interviews revealed the positive effects of teachers' online WCF on students' self-regulation in writing, especially those with mid- and low-proficiencies. Results from this study provided an indication of the effectiveness of online WCF in developing L2 learners' self-regulation. They also gave insight into how language proficiency levels affect learners' perception of online feedback and its impact on self-regulated learning development.