Long-chain n-3 polyunsaturated fatty acid (PUFA) consumption in maternal and infants has been positively associated with cognitive and visual development. Tails even meta-analysis showed mixed results. To evaluate the effects of maternal and infant n-3 PUFA supplementation on childhood psychomotor and cognitive development, PubMed, Embase, the Cochrane Library, PsycINFO and clinicaltrials.gov were searched. Randomized controlled trials were included to evaluate the effect on child cognitive and psychomotor outcomes of n-3 PUFA supplementation in mothers or infants (age ≤ 2 years). Findings were pooled with mean differences (MD) with 95% confidence intervals (95% CIs). Heterogeneity was explored using I2 and subgroup analyses, stratified for maternal (pregnancy and/or lactation) and infant (preterm infant and term infant). We identified 47 articles, with 14 trials on mothers and 33 on infants. Pooled results showed that infants' mental development index (MDI) increased with n-3 PUFA supplementation (MD = 2.91, 95% CI: 1.32-4.51, I2 = 65.1%). Subgroup analysis of MDI also demonstrated a benefit in preterm infants (MD = 4.16, 95% CI: 1.40-6.93, I2 = 49.5%) and term infants (MD = 2.28, 95% CI: 0.27-4.29, I2 = 70.1%). No significant association was found in subgroup analyses of supplementation to mothers during pregnancy or lactation period. Supplementation did not increase the psychomotor development index (PDI) in the mother or infant group. Language composite score increased for infants whose mothers accepted supplementation in pregnancy or breastfeeding (MD = 8.57, 95% CI: 5.09-12.04, I2 = 70.2%). The cognitive composite score did not improve in any subgroup. Intelligence Quotient (IQ) increased in the infants' group with n-3 PUFA supplementation (MD = 2.54, 95% CI: 0.45-4.63, I2 = 66.0%). Furthermore, IQ in term infants also improved (MD = 2.91, 95% CI: 0.24-5.57, I2 = 69.2%). The funnel plot and Egger's test confirmed no publication bias in any endpoints. Supplementation with n-3 PUFA during pregnancy or breastfeeding in mothers has increased language abilities. Furthermore, direct supplementation in term infants can improve intelligence in later childhood. However, insufficient evidence supports the claim that supplementation improves cognitive abilities.© 2024 The Author(s). Maternal & Child Nutrition published by John Wiley & Sons Ltd.