Online feedback is frequently implemented during second/foreign language (SL/FL) writing tasks and assessments. This meta-analysis investigates the effectiveness of online feedback in SL/FL writing. After careful screening and the application of inclusion and exclusion criteria, this study synthesizes the results of 17 primary studies reporting on students' English SL/FL writing quality after online feedback. The studies involved 1568 students, and the results indicate a Hedges' g effect size of 0.753 for the effectiveness of written feedback in general. Online feedback from teachers/instructors produces a larger effect size (g = 2.248) than online peer feedback (g = 0.777) and online automated feedback (g = 0.696). It was also found that educational levels and task genre mitigate the impact of online feedback on writing quality. Overall, the findings contribute to a better understanding of the impact of online feedback on ESL/EFL writing and provide insights into online ESL/EFL writing instruction.