Background: The early years have been identified as a critical period during which sedentary habits may be established, as this behavior appears to track throughout life. Objective: The purpose of this systematic review and meta-analysis is to summarize the current literature on the prevalence of sedentary behavior in children aged 1-5.99 years, reporting differences between boys and girls, weekdays and weekend days, childcare hours and nonchildcare hours, and between time spent indoors and outdoors while children attended childcare. Methods: Five databases were searched until 26.10.2017 and meta-analyses were conducted to estimate prevalence and calculate mean differences in prevalence between groups. Results: Fifty studies representing 14 598 children (2-5.99 years) were included. Children spent 51.4% of their waking time in sedentary behaviors. Boys spent less time being sedentary than girls (estimate difference = −1.4%; 95%CI = −2.0: −0.7; P <.001). No significant differences were found between weekdays/weekend days (estimate difference = −0.4; 95%CI = −2.0: 1.2; P =.61) nor between childcare hours/nonchildcare hours (estimate difference = %; 95%CI = −0.9: 6.6; P =.136). While attending childcare centers, children were more sedentary indoors than outdoors (estimate difference = 14.4%; 95%CI = 11.8: 16.9; P <.001). Conclusions: Our results suggest that young children spend a significant portion of their waking hours in sedentary behaviors. While at childcare, young children accumulated more sedentary behavior indoors than outdoors. Girls were more likely to be more sedentary than boys. No significant differences were found between weekdays and weekends, or between childcare and nonchildcare hours. There is a need for higher-quality studies with strong designs, using age and device appropriate cut-off points, to improve evidence-base and to better establish prevalence of sedentary behavior in young children.