Background: Blended learning programs in Kindergarten through Grade 12 (K-12) classrooms are growing in popularity; however, previous studies assessing their effects have yielded inconsistent results. Further, their effects have not been completely quantitatively synthesized and evaluated. Objectives: The purpose of this study is to synthesize the overall effects of blended learning on K-12 student performance, distinguish the most effective domains of learning outcomes, and examine the moderators of the overall effects. Methods: For the purpose, this study conducted a meta-analysis of 84 studies published between 2000 and 2020, and involved 30,377 K-12 students. Results and Conclusions: Results revealed that blended learning can significantly improve K-12 students' overall performance [g = 0.65, p < 0.001, 95% CI = (0.54-0.77)], particularly in the cognitive domain [g = 0.74, p < 0.001, 95% CI = (0.61-0.88)). The testing of moderators indicates that the factors moderating the impact of blended learning on student performance in these studies included group activities, educational level, subject, knowledge type, instructor, sample size, intervention duration and region. Implications: The results indicate that blended learning is an effective way to improve K-12 students' performance compared to traditional face-to-face (F2F) learning. Additionally, these findings highlight valuable recommendations for future research and practices related to effective blended learning approaches in K-12 settings.