<p>Some studies have evidenced that metacognitive interventions have the potential to enhance academic achievement and reduce educational inequalities among primary and secondary school students. However, no intervention to date has investigated whether improving metacognition in preschoolers could reduce these inequalities. This is particularly critical, as previous studies reported that the effect of family socioeconomic status (SES) on academic abilities is mediated by metacognitive abilities. In the present study, we tested the effect of a classroom intervention aiming at fostering metacognition on educational inequalities in kindergarteners. We collected data from 344 children aged 5 to 6 on their family SES, and metacognition, language and mathematics abilities at pre-test, immediate post-test, and a three-month follow-up. Consistent with a previous study, we found that metacognition mediated the relation between family SES and academic abilities in language and mathematics. Moreover, the intervention improved metacognitive knowledge and arithmetical operations of the kindergarteners of the experimental group (<i>n</i> = 172), as compared to those in the passive control group (<i>n</i> = 172). Importantly, the results suggest that low-SES children benefited more from the intervention at the follow-up when considering grammar abilities. This study is the first to demonstrate delayed post-test effects, transfers to academic learning, and potential SES-differentiated effects of a metacognitive intervention in this age group. These findings highlight the potential of metacognitive interventions to address early educational inequalities and emphasize the need for further research and policy initiatives, particularly in the area of teacher training.</p>