The COVID-19-related economic crisis and food- and health-system disruptions threaten to exacerbate undernutrition in low- and middle-income countries (LMIC). We used three modelling tools (MIRAGRODEP, Lives Saved Tool, and Optima Nutrition Tool) to estimate impacts on child stunting, wasting, and mortality, maternal anemia, children born to low BMI women, and future productivity losses for three scenarios across 2020–2022 (n = 118 LMICs). We also estimated the additional cost of mitigation for six nutrition interventions to maximize non-stunted and alive children. By 2022 COVID-19 could result in an additional 9.3 million wasted and 2.6 million stunted children, 168,000 additional child-deaths 2.1 million maternal anemia cases, 2.1 million children born to low BMI women and US$29.7 billion future productivity losses due to excess stunting and child mortality. An additional $1.2 billion per year is needed to mitigate these effects. Governments and donors must maintain nutrition as a priority, continue to support resilient systems, and ensure efficient use of new and existing resources.