Outdoor playtime has been highly correlated with moderate-to vigorous-intensity physical activity (MVPA), while also being associated with decreased sedentary time. This systematic review and meta-analysis sought to examine the physical activity levels and sedentary time of young children (2-5years) during outdoor play periods at centre-based childcare. Eight online databases were searched for peer-reviewed, English-language, original research. Two reviewers independently extracted data (study design, participants, method of measurement, and physical activity and/or sedentary time of participants). Time spent in outdoor activity intensities were converted to percentage of time, and averaged across the two measurement types: accelerometry and observational. Of the 26 included articles, 11 used accelerometry, 13 employed direct observation, and 2 adopted a combination of both measurement methods. Average percentage of time engaged in MVPA and sedentary pursuits ranged from 6.7% to 43% and 23.2% to 63.5% by accelerometry, and 2% to 53.9% and 46.1% to 80.2% by observation, respectively. Total physical activity (only measured by accelerometry) ranged from 23.2% to 63.5% of outdoor playtime. Meta-analysis of 11 accelerometer studies found that children spent a mean (95% CI) of: 13.99% (10.35-17.63) of outdoor playtime in MVPA, 43.77% (32.95-54.58) in TPA, and 52.81% (44.1-61.51) in sedentary time. Overall, young children have the potential to be highly active during outdoor play sessions at centre-based childcare; however, many children still engage in substantial amounts of sedentary time. Future research should explore the frequency and/or duration of outdoor time which best support young children's physical activity and limits sedentary time in childcare.