Zinc is an important micronutrient for humans, and zinc deficiency among schoolchildren is deleterious to growth and development, immune competence, and cognitive function. However, the effect of zinc supplementation on cognitive function remains poorly understood. The purpose of our study was to evaluate the effect of oral zinc supplementation (5 mg Zn/day for 3 months) on the Full Scale Intelligence Quotient (FSIQ), Verbal Intelligence Quotient (VIQ), and Performance Intelligence Quotient (PIQ) using a Wechsler Intelligence Scale for Children (WISC-III). We studied 36 schoolchildren aged 6 to 9 years (7.8 ± 1.1) using a nonprobability sampling method. The baseline serum zinc concentrations increased significantly after zinc supplementation (p < 0.0001), with no difference between sexes. Tests were administered under basal conditions before and after zinc supplementation, and there was no difference in FSIQ according to gender or age. The results demonstrated that zinc improved the VIQ only in the Information Subtest (p = 0.009), although the supplementation effects were more significant in relation to the PIQ, as these scores improved for the Picture Completion, Picture Arrangement, Block Design, and Object Assembly Subtests (p = 0.0001, for all subtests). In conclusion, zinc supplementation improved specific cognitive abilities, thereby positively influencing the academic performance of schoolchildren, even those without marginal zinc deficiency.