Fish provide essential nutrients for the critical window of growth and development in the first 1000 days of life and are thus an attractive option for inclusion in both nutrition-sensitive and nutrition-specific programming. We conducted a systematic review of the evidence for fish and fish-based products for nutrition and health outcomes during the first 1000 days of life in low- and middle-income countries, searching the peer-reviewed and grey literature from 1999 to 2020. Databases searched included PubMed, Web of Science, EMBASE, Proquest, and the Clinical Trials repository. Our search returned 1,135 articles, 39 of which met the inclusion criteria. All studies were dual evaluated for risk of bias (RoB). Of the included studies, 18 measured maternal health and nutrition outcomes and 24 measured infant/child outcomes (three measured both). Our search uncovered ten impact evaluations, all of which measured consumption of fish or fish-based complementary food products in children aged 6–24 months of age. We did not find strong evidence for fish consumption in children improving child growth from the impact evaluations, however the studies were highly heterogeneous in their design and were likely underpowered to detect an effect. Results from observational studies were mixed but provided evidence that adding fish to maternal and child diets is associated with improved nutrition outcomes, such as reducing the risk of anemia and improving vitamin D status. Given the nutrient richness of fish, and the fact that production is often more environmentally friendly compared with other animal source foods, more robust evidence is needed on the role of fish consumption in nutrition interventions to inform policy and programming recommendations in low- and middle-income countries.