Background: Child growth assessment serves as a tool for assessing children's health and nutritional condition. Longitudinal studies have statistical power in inspecting growth changes. Objectives: This study aimed to examine the longitudinal growth differences and determinants of children's body weight in Ethiopia. Methods: Longitudinal data were obtained from the Young Lives cohort study conducted in Ethiopia from 2002 to 2016. The study involved 1,665 Ethiopian children who were followed from age 1 to 15 years. The obtained data were analyzed using a fractional polynomial mixed-effect model. Results: Children's weight gain followed non-linear trajectories. There were considerable differences in children's body weight across different regions of Ethiopia. Children in Addis Ababa, Oromiya, and SNNPR had much faster weight growth than Tigray children. There was no substantial difference in the children's rate of weight gain between Tigray and the Amhara region. Boys had a faster rate of change in weight gain than girls. Conclusions: Controlling for other factors, gender, residence, region, parental education, household size, wealth, and mother's age had significant influences on the longitudinal weight growth of children. However, safe drinking water, sleeping time, and father's age were not related to the weight trajectory over time.