Project-based learning is generally considered an alternative to traditional, teacher-led instruction. However, there is a noticeable lack of meta-analyses with regard to determining its overall effects on students' academic achievement, and what study features may moderate the impacts of project-based learning. This study thus performed a meta-analysis to synthesize existing research that compared the effects of project-based learning and those of traditional instruction on student academic achievement. Forty-six effect sizes (comparisons) extracted from 30 eligible journal articles published from 1998 to 2017 were analyzed, representing 12,585 students from 189 schools in nine countries. The results showed that the overall mean weighted effect size (d+) was 0.71, indicating that project-based learning has a medium to large positive effect on students' academic achievement compared with traditional instruction. In addition, the mean effect size was affected by subject area, school location, hours of instruction, and information technology support, but not by educational stage and small group size.