Reflective thinking is a must-have skill to connect the knowledge obtained with previous knowledge and can be seen from learning achievement. This study aims to prove and determine the relationship between reflective thinking and learning achievement and its effect size. This study used a quantitative meta-analysis method. Reflective thinking is the independent variable and learning achievement is the dependent variable. The data sources were obtained from online database searches on Google Scholar and international journal platforms from 2012 to 2021. Based on the search, 22 research publications met the predetermined criteria through a strict screening. Quantitative meta-analysis with correlation meta-analysis type was used to analyze the data. The software used was JASP 0.8 4.0. The results showed that this research Ho is rejected. It can be concluded that there was a significant relationship between reflective thinking and student achievement (z = 8.139; p < 0.001; 95%CI [0.400; 0.654]). The effect of reflective thinking on student achievement was in the medium category (r[subscript RE] = 0.527). The findings are consistent with those of previous research on reflective thinking skills and learning achievement.