Background: Extreme heat may discourage physical activity of children while shade may provide thermal comfort. The authors determined the associations between ambient temperature, shade, and moderate to vigorous physical activity (MVPA) of children during school recess. Methods: Children aged 8–10 (n = 213) wore accelerometers and global positioning system monitors during recess at 3 school parks in Austin, Texas (September–November 2019). Weather data originated from 10 sensors per park. The authors calculated shade from imagery using a geographic information system (GIS) and time-matched physical activity, location, temperature, and shade data. The authors specified piecewise multilevel regression to assess relations between average temperature and percentage of recess time in MVPA and shade. Results: Temperature ranged 11 °C to 35 °C. Each 1 °C higher temperature was associated with a 0.7 percentage point lower time spent in MVPA, until 33 °C (91 °F) when the association changed to a 1.5 lower time (P < .01). Each 1 °C higher temperature was associated with a 0.3 percentage point higher time spent under shade, until 33 °C when the association changed to a 3.4 higher time (P < .001). At 33 °C or above, the direct association between shade and MVPA weakened (P < .05), with no interaction effect above 33 °C (P > .05). Children at the park with the most tree canopy spent 6.0 percentage points more time in MVPA (P < .01). Conclusions: Children engage in less MVPA and seek shade during extreme heat and engage in more MVPA in green schoolyards. With climate change, schools should consider interventions (eg, organizing shaded play, tree planting) to promote heat safe MVPA.