The metaverse offers unique opportunities for K-12 students to learn STEM subjects in online and blended settings, providing interactive and authentic experiences. Although previous studies have produced mixed findings on the influence of the metaverse on perceptions and academic achievement of online and blended STEM learning, it is necessary to study the overall impact of the metaverse on K-12 students' STEM learning and how the related factors, such as technologies used, pedagogical approaches, learners' education levels, and domains of learning, modulate STEM learning outcomes. Therefore, the study applied a meta-analysis method to assess the influence of the metaverse on K-12 students' academic achievement and perceptions of STEM in online and blended learning settings, while also examining the moderating effects in 49 studies. The effect sizes for academic achievement and perceptions were estimated to be 0.821 (indicating a great impact) and 0.575 (indicating a moderate impact), respectively. The moderation analysis showed that the learning domains and learners' education level were significant moderators of the influence of the metaverse on their academic achievement, with elementary students showing greater improvement in science subjects. Moreover, the technologies used and pedagogical approaches played a pivotal role as moderators in shaping students' perceptions of the impact of the metaverse, with VR and AR technologies and inquiry-based learning being particularly effective in terms of enhancing learner perceptions. Thus, it is recommended for teachers to actively utilize the metaverse for STEM teaching, while considering their students' educational level, learning domain, appropriate technology use, and pedagogical approaches.