Background: Self-reports are commonly used to assess physical activity in children. Existing self-reports for physical activity have not been validated for use among primary school children in, Tanzania. In order to understand if primary school children can accurately report their physical activity, we examined the validity of self-reported physical activity against accelerometer measured physical activity. Methods: A community based cross-sectional study was conducted from May to July, 2018. Four primary schools were conveniently selected in Moshi municipal and Moshi rural districts in Kilimanjaro, Tanzania and from these 51 children aged 9 – 11 years were randomly selected. Self-reported questionnaire was used to collect physical activity related variables. In addition, children wore accelerometers for seven consecutive days to capture physical activity movements. Spearman’s rank test and Bland Altman plots were used for assessing validity and agreement between self-reports and accelerometer moderate to vigorous physical activity (MVPA). Results: The mean age of the study participants was 10 (SD=0.8) years and 32 (63%) were girls. A positive significant correlation was found between self-reports and accelerometer MVPA (rho=0.36, p=0.009). Accelerometer had higher mean MVPA 408 (SD = 66) compared to self-reports 261 (SD = 179). Children who reported walking to school had higher MVPA for both accelerometer and self- reports compared to children who use other means of transport to school, e.g. school buses (p < 0.001). Conclusions: This study found a positive significant correlation between self-reports and accelerometers. Self-reports are prone to errors due to recall bias, and this interferes their validity. More research is needed to develop better self-reported measures with specific activities which can easily be recalled by children. Also, researchers have to be aware of self-reports validity limitation.