Background: The n-3 (omega-3) long-chain polyunsaturated fatty acid (LC-PUFA) docosahexaenoic acid (DHA) is essential for optimal brain development. There is a lack of evidence on the effect of postnatal n-3 LC-PUFA supplementation on child development in low-income countries. Objective: We evaluated the efficacy of fish-oil supplementation through lactation or complementary food supplementation on the development of children aged 6-24 mo in rural Ethiopia. Methods: We conducted a double-blind randomized controlled trial of n-3 LC-PUFA supplementation for 12 mo using fish-oil capsules [maternal intervention: 215 mg DHA + 285 mg eicosapentaenoic acid (EPA)] or a fish-oil-enriched complementary food supplement (child intervention: 169 mg DHA + 331 mg EPA). In total, 360 pairs of mothers and infants aged 6-12 mo were randomly assigned to 4 arms: maternal intervention and child control, child intervention and maternal control, maternal and child intervention, and maternal and child control. Primary outcomes were overall developmental performance with the use of a culturally adapted Denver II test that assesses personal-social, language, fine-motor, and gross-motor domains and social-emotional developmental performance using the Ages and Stages Questionnaire: Social Emotional at baseline and at 6 and 12 mo. We used mixed-effects models to estimate intervention effects on developmental performance over time (intervention x time interaction). Results: The evolution in overall and social-emotional developmental performance over time did not differ across study arms (intervention x time: F = 1.09, P = 0.35, and F = 0.61, P = 0.61, respectively). Effects did not change after adjustment for child age, birth order, and nutritional status; maternal age and education; wealth; family size; and breastfeeding frequency. Children's developmental performance significantly decreased during study follow-up (beta: -0.03 SDs/mo; 95% CI: -0.04, -0.01 SD/mo; P < 0.01). Conclusions: n-3 LC-PUFA supplementation does not affect overall or social-emotional development of children aged 6-24 mo in a low-income setting. Follow-up of the cohort is recommended to determine whether there are long-term effects of the intervention.