Formative assessment in K-12 education has been a notable teaching and learning focus area in schools over the last 20 years, as evidenced by numerous recent systematic reviews and meta-analyses investigating and summarizing the evidence for formative assessments' effectiveness and sustainability. This umbrella review systematically reviews meta-analyses investigating the effects of formative assessment on learning, summarizes the current findings, and assesses the quality and risk of bias in the published meta-analyses. Meta-analyses were identified using systematic literature searches in the following databases: Scopus, ERIC, Academic Research Complete, ProQuest, APA PsycArticles, SPORTDiscus, Web of Science, and Humanities International Complete. Thirteen meta-analyses, each of which examined the effects of formative assessment on learning in K-12 students, were included in this umbrella review. The review considered evidence for the potential effectiveness of using formative assessment in class with primary and secondary school students. Formative assessment was found to produce trivial to large positive effects on student learning, with no negative effects identified. The magnitude of effects varied according to the type of formative assessment. The 13 included meta-analysis studies showed moderate (n = 10), high (n = 1), and low (n = 2) methodological quality (AMSTAR-2), although the robustness of the evidence (i.e., GRADE analysis) was very low (n = 9), low (n = 3), and moderate (n = 1). These findings offer valuable insights for designing and implementing different types of formative assessment aimed at optimizing student learning and ensuring the sustainability of assessment practices. However, the low-to-very-low certainty of the available evidence precludes robust recommendations regarding optimal formative assessment strategies for learning in K-12 students.