The aim of this systematic review was to summarize ECEC environmental correlates of weight status in children under the age of 6. years. Six databases (PubMed, PsycINFO, CINAHL, SPORTDiscus, Scopus, and Web of Science) were searched until March 2017. Observational studies examining the relationship between ECEC environmental characteristics and weight status in children aged 0-6. years were included. Data was extracted using a predesigned form. Eight studies, representing 4862 children, met the inclusion criteria. Twenty-two environmental characteristics were identified and classified into four domains (physical, political, economic, and sociocultural); of these, six correlates were found. 'Active environment' 'sedentary opportunities', 'active play time', 'high sugar and high fat served', 'educators' weight' and 'educators' habitual physical activity level' were associated with weight status in young children. However, for most environmental characteristics examined, strong evidence is not available yet, due to variations across studies on the measures of environmental characteristics and analytical methodologies. Stronger empirical evidence in greater quantity is needed. Future studies in this area are recommended to investigate the environmental influence using an ecological approach and to examine the potential mediators, with a focus on the settings of family-based centres and samples representing toddlers and/or infants.