Substantial research has explored the connection between children's spontaneous focusing on numerosity (SFON) and their current and later mathematical achievement. However, the findings have been inconsistent, and no comprehensive investigation has yet been conducted. This meta-analysis examines the relation between SFON and mathematics performance in preschool and school-age students and identifies potential moderators, including SFON measurement methods, SFON scoring methods, number size in SFON tasks, domains of mathematics skills, temporal relations, time lags, and age. Based on 171 effect sizes from 35 independent samples, involving 6604 participants across 29 studies, results revealed a significant positive correlation between SFON and mathematics performance (r = 0.274), with bidirectionality and longitudinal stability. The correlation remained significant after controlling for working memory, nonverbal intelligence, and inhibition, respectively. Moderation analyses showed stronger correlations with mathematics performance for behavioral-based tasks and scoring methods that consider both accuracy and quantifying acts. SFON tasks involving smaller numbers (less than ten) yielded stronger correlations with mathematics performance. Temporal relations emerged as a significant moderator, with a stronger longitudinal link from mathematics performance to SFON compared to the reverse. Age was another significant moderator, with the link strengthening with age. Interaction effects among moderators were also observed. Theoretical implications of these findings regarding the SFON-mathematics performance relation are discussed, and practical implications for comprehensive assessment of children's SFON.