Conducted in Italy, this study was designed to test the impact on an in-school chess intervention on the mathematics performance of 3rd through 7th grade students. The treatment group was composed of 412 students and the control group was composed of 156 students. Students who participated in the in-school chess intervention were also invited to complete additional chess practice through computer-assisted training online. Each group was further subdivided and a portion of students in each group took a pre-test composed of OECD-PISA mathematics items. All students completed the same assessment following the conclusion of the chess intervention. For students who took both pre- and post-intervention assessments, individual student gains in mathematics were calculated. Then, the gains of the experimental groups were compared to the gains of the control group using univariate ANOVA. The effect size in mathematics was 0.421 standard deviation units. The results were statistically significant. The study was eligible for inclusion in the systematic literature review and categorized as Tier III. Assignment to treatment and control groups was not randomized, and no matching of students in the treatment and control groups based on student characteristics was conducted.