Objective: To evaluate the impact of the duration of breastfeeding on the intelligence of children. Materials and Methods: Eight electronic databases were searched to identify studies that investigated the impact of breastfeeding on the intelligence of children. Data were pooled, and the ratio of means (RoM) and the corresponding 95% confidence interval (95% CI) were calculated using a pairwise meta-analysis and network meta-analysis. Risk of bias was assessed using a tool developed by the CLARITY group. Data were analyzed using R version 3.5.1. Results: A total of 16 studies with 9,162 subjects were included in the review. Half of the studies were at low risk of bias. A meta-analysis indicated that breastfed children had a score 1.04-fold higher in intelligence tests compared with those that had never been breastfed (RoM: 1.04, 95% CI: 1.02-1.06, p < 0.05). Evidence from a network meta-analysis indicated that breastfeeding for <= 6 months resulted in score 1.04-fold higher in intelligence tests (RoM: 1.04, 95% CI: 1.03-1.06, p < 0.05) and children breastfed for >6 months had a score 1.06-fold higher (RoM: 1.06, 95% CI: 1.05-1.08, p < 0.05) than children that had never been breastfed. Thus, breastfeeding for >6 months demonstrated a slightly higher score than breastfeeding for <= 6 months (RoM: 1.02, 95% CI: 1.00-1.04, p < 0.05). Conclusion: Breastfeeding could significantly improve the intelligence of children, with a duration of >6 months showing a slight but significantly higher intelligence score than for <= 6 months.