Epistemic cognition, defined as the ways that people acquire, justify, and use knowledge, has been a prominent area of scholarship in educational psychology for nearly 50 years. Researchers have argued that epistemic cognition is a key predictor of many 21st century learning outcomes including critical thinking, scientific literacy, and historical thinking, among others. Despite a large volume of quantitative empirical research on epistemic cognition and academic achievement, there has been no published systematic analysis of this literature. We conducted a meta-analysis of 132 nonexperimental studies in this literature, and found epistemic cognition, as measured predominantly in terms of beliefs, was positively correlated with academic achievement, r = 0.162, p < 0.001, indicating a small but meaningful relationship. Moderator analyses revealed models and instruments focusing on development and justification of knowledge had higher correlations with academic achievement than those focused on constructs related to authority. We found evidence supporting domain-general, domain-specific, and topic-specific conceptualizations of epistemic cognition, and stronger correlations when the specificity of the epistemic cognition measure matched the specificity of the achievement measure. Conceptual knowledge and argumentation performance were more strongly associated with epistemic cognition than declarative or procedural knowledge performance. Finally, we found epistemic cognition predicted academic achievement from elementary school through graduate school, suggesting the importance of studying epistemic cognition across all educational levels. These findings have direct implications for honing epistemic cognition theory, creating better measures of epistemic cognition, and developing effective educational interventions on this critical topic. Educational Impact and Implications Statement: The findings from this meta-analysis show a small but important relationship between epistemic cognition and academic achievement. Epistemic cognition seems most strongly related to conceptual understanding and argumentation in academic domains. Therefore, educators who wish to help their students develop such knowledge and skills should investigate ways of fostering students' epistemic cognition. Our study points to several key foci for such work, including teaching students the ways knowledge develops and is justified within academic domains.