Background Although controversial, lower maternal intake of n-3 polyunsaturated fatty acid (PUFA) during pregnancy and lower levels of omega-3 PUFA in serum phospholipids during childhood have been related to obesity. The main source of omega-3 PUFA is fatty fish in the diet. Objectives To assess the relationship between overweight/obesity and the intake of fatty fish in maternal diet during pregnancy and in children up to 8 years of age. Methods The prospective cohort All Children in South-East Sweden (ABIS) followed babies from birth to 8 years of age. A total of 6749 children at 5 years of age (boys 52.6%) and 3017 children at 8 years (boys 52.3%) participated. A “fatty-fish index” was constructed on the basis of self-reports of nutritional habits. Results The prevalence of overweight and obesity in children at 5 years were 12.9% and 4.2%, respectively. At 8 years, 12.2% of the children presented overweight and 2.3% obesity. Girls were more affected than boys by overweight/obesity. A higher fish index during pregnancy was not related to overweight/obesity in the children, whereas a higher fish index in the children during the first years of life was related to obesity at 5 and 8 years of age. This relationship disappeared in a multivariable analysis. Maternal body mass index (BMI), maternal education, maternal smoking during pregnancy, birth weight, and physical activity all remained related to overweight/obesity at both 5 and 8 years of age. Conclusion No relationships were found between a lower intake of fatty fish in the diet, neither in mothers during pregnancy nor in early childhood, and increased risk of overweight/obesity.