Math anxiety and math achievement are reciprocally related, a relationship that is likely to affect individuals’ agency and educational and career trajectories. Several meta-analyses have synthesized the extensive research on this relationship by integrating the correlations reported in primary studies (i.e., aggregated data; AD). However, the generalizability of these meta-analytic findings is limited because (a) most primary studies were based on small, nonrepresentative samples of college students or came from Western countries and (b) the search methods they applied largely neglected results and individual participant data (IPD) from international large-scale assessments (ILSAs). Moreover, previous AD meta-analyses could not provide conclusive evidence of moderating effects (e.g., gender) due to several methodological issues, including low statistical power and ecological bias. This meta-analysis advances knowledge about the relationship between math anxiety and math achievement by integrating systematically selected IPD from ILSAs (309 samples; total N = 1,884,248; 88 countries) with AD (285 samples; total N = 83,795; 52 countries) from the most extensive meta-analysis on this topic to date by Barroso et al. (2021). The meta-analysis of these IPD and AD indicated a robust negative relationship between math anxiety and math achievement (r = –.28) that was moderated by several factors, for example, age, math anxiety components, and math subdomains. Importantly, meta-analyzing the IPD provided new insights into moderating effects of individuals’ gender, math achievement on the individual and sample levels, and between-country differences. In summary, synthesizing IPD and AD generated the most reliable, generalizable, and nuanced evidence for research, practice, and policy.