The aim of the present study was to advance knowledge of the secular changes in the prevalence of excess weight (overweight and obesity) and body composition of children living in different localities of La Plata district (Buenos Aires province, Argentina) over the last two decades. For this purpose, anthropometric data and information on the socioeconomic and environmental context of residence of schoolchildren aged 4.0-12.9 years collected during 2005-2009 (Cohort 1, n = 1 233) and 2016-2019 (Cohort 2, n = 1 499) were analysed and compared. From a secular perspective, the current analysis showed that the socioeconomic and environmental conditions of families living in La Plata district have deteriorated in recent decades. In this context, child excess weight increased by 13% and body composition was altered, mainly as a result of an increase in fat mass, without observing changes in the muscle component.