Teachers’ professional development is crucial for effective classroom practice. Due to its advantages, many teachers have participated in online professional development (OPD) in recent years. Numerous studies have investigated the effectiveness of OPD either on the teacher, their classroom, or the student level. However, a comprehensive meta-analysis of these studies is missing. This meta-analysis summarizes findings from 85 quantitative studies with a pre-posttest design. The results indicate large effects of OPD participation on the teacher level (d = 0.66), a medium to large effect on the classroom level (d = 0.59), and a small effect on the student level (d = 0.21). Additionally, we found that the core feature of PD activities, such as collective participation, was positively associated with the effect of the classroom level. Furthermore, we found that studies employing a control-group design reported significantly lower effect sizes on teacher-level outcomes than studies using a within-subject design. At the student level, studies with asynchronous OPD formats reported significantly smaller effect sizes than studies with synchronous OPD formats. Our results suggest that OPD can have substantial simultaneous effects on different school system levels and could be fostered more systematically to enhance school development and improvement processes.