Background: Online learning has become more prevalent over the past three decades, especially during the COVID-19 pandemic. Educators and scholars have increasingly emphasized the significance of self-directed learning (SDL) on successful learning outcomes in online learning environments. Objectives: The purpose of this study was to synthesize the research on the effects of SDL on learning achievement in online learning environments. Methods: This study synthesized the research findings on the effects of SDL on learning achievement in online learning environments using a meta-analysis method. We analysed 152 samples from 43 studies that have reported the effects of SDL on learning achievement or correlations in online learning. Results and Conclusions: The overall effect size of SDL on learning achievement in online learning showed a medium effect size (g = 0.508). The dimensions of SDL moderated the effects of SDL on learning achievement, and the effects of self-management were statistically smaller than motivation and self-monitoring. The types of learning achievement, learning domains, and participant levels did not moderate the effects of SDL on learning achievement. However, the effects of SDL on learning achievement in the cognitive domain were significantly smaller than the effects of SDL in the affective domain. What Are the 1 or 2 Major Takeaways from the Study? This study confirmed the importance of SDL in online learning environments. SDL can be effective for any learner group, not just for adult learners, in online learning environments. Online instructors should facilitate students' self-management to obtain successful learning achievement.