Abstract Objectives Meeting infant needs requires complementing breastmilk with the gradual introduction of solid foods after 6 months. Existing studies have limited evidence on the availability and cost of complementary foods needed during the 6–23 month window when millions of children experience irreversible harm from inadequate diets. Methods Using food prices from the World Bank on 374 items potentially used for infant foods and available for sale in 31 low-income countries, food composition data, and dietary reference intakes (DRIs), we compute least-cost diets for children between 6–23 months. We further used data for food yield and nutrient retention factors and introduced volume constraints. DRIs differ for children aged 6–8 months, 9–11 months, and 12–23 months. We used lower and upper bounds on 25 different nutrients plus fiber and sodium. Our primary specification (Scenario I) considers only total protein and fats, while a more restrictive Scenario II also considers 11 essential amino acids and fatty acids. Results The cost of complementary foods to replace breastmilk is $1–2/day at 6–8 months when small quantities of high-cost ingredients are needed, then declines to below $/1day at 9–11 months when lower-cost foods can be used before rising above $1/day again as children grow. Increased breastfeeding substitutes for complementary foods to a limited degree, as shown by the small decline in cost as breastmilk intake rises, and there is a clear threshold beyond which sufficiently nutrient-dense ingredients are very expensive or entirely unavailable. Average intake recommendation levels of breastmilk measured in low-income countries are close to that which makes complementary feeding least expensive, except in Scenario II for the youngest children. Conclusions We find that sufficient nutrients from complementary foods for infants from 6 to 23 months are not affordable for many households in these countries, implying a need for targeted assistance, especially in settings where household incomes are below the global poverty line of $1.90/person/day. Funding Sources This study was funded by the Bill & Melinda Gates Foundation through the project Advancing Research on Nutrition and Agriculture (AReNA), Phase II (Investment ID: OPP1177007), jointly with the project Food Prices for Nutrition (INV-016158) funded by the Bill & Melinda Gates Foundation and UKAid.