This study aims to determine the effect of STEM applications on mathematics achievement using the meta-analysis method. For this purpose, postgraduate theses in Türkiye were analysed. As a result of the review, 25 postgraduate theses related to the topic were initially identified, and data from 14 theses that met the inclusion criteria were included in the meta-analysis. Data analyses were carried out using the Comprehensive Meta-Analysis statistical program. While applying the meta-analysis method, homogeneity test was performed by calculating the effect sizes of the studies. Since the studies showed a heterogeneous structure as a result of the homogeneity test, the random effects model was used to calculate the overall effect size. Hedges's g coefficient was used to determine the effect sizes. Funnel plot and Rosenthal's safe N statistic were used to examine publication bias. The classification of Thalheimer and Cook (2002) was used to interpret the effect sizes obtained as a result of the analyses. In the study, it was determined that the effect sizes of all studies included in the meta-analysis were positive. Only two of the studies had a small effect, while the other studies showed at least a medium effect. The overall effect size value for mathematics achievement was found to be 0.901. Based on these results, it can be said that STEM activities have a positive and large effect on increasing mathematics achievement.