question: Previous studies showed that the coronavirus disease 2019 (COVID-19) lockdown imposed changes in adults' lifestyle behaviors; however, there is limited information regarding the effects on youth. The COV-EAT study aimed to report changes in children's and adolescents' lifestyle habits during the first COVID-19 lockdown and explore potential associations between changes of participants' lifestyle behaviors and body weight. An online survey among 397 children/adolescents and their parents across 63 municipalities in Greece was conducted in April-May 2020. Parents self-reported changes of their children's lifestyle habits and body weight, as well as sociodemographic data of their family. The present study shows that during the lockdown, children's/adolescents' sleep duration and screen time increased, while their physical activity decreased. Their consumption of fruits and fresh fruit juices, vegetables, dairy products, pasta, sweets, total snacks, and breakfast increased, while fast-food consumption decreased. Body weight increased in 35% of children/adolescents. A multiple regression analysis showed that the body weight increase was associated with increased consumption of breakfast, salty snacks, and total snacks and with decreased physical activity. The COV-EAT study revealed changes in children's and adolescents' lifestyle behaviors during the first COVID-19 lockdown in Greece. Effective strategies are needed to prevent excessive body weight gain in future COVID-19 lockdowns.