Background: Beginning March 2020, the COVID-19 pandemic has disrupted different aspects of life. The impact on children's rate of weight gain has not been analysed. Methods:In this retrospective cohort study, we used United States (US) Electronic Health Record (EHR) data from Optum® to calculate the age-and sex-adjusted change in BMI ( BMI adj ) in individual 6to-17-year-old children between two well child checks (WCCs). The mean of individual BMI adj during 2017-2020 was calculated by month. For September-December WCCs, the mean of individual BMI adj (overall and by subgroup) was reported for 2020 and 2017-2019, and the impact of 2020 vs 2017-2019 was tested by multivariable linear regression. Findings: The mean [95% Confidence Interval -CI] BMI adj in September-December of 2020 was 0 •62 [0 •59,0 •64] kg/m 2 , compared to 0 •31 [0 •29, 0 •32] kg/m 2 in previous years. The increase was most prominent in children with pre-existing obesity (1 •16 [1 •07,1 •24] kg/m 2 in 2020 versus 0 •56 [0 •52,0 •61] kg/m 2 in previous years), Hispanic children (0 •93 [0 •84,1 •02] kg/m 2 in 2020 versus 0 •41 [0 •36,0 •46] kg/m 2 in previous years), and children who lack commercial insurance (0 •88 [0 •81,0 •95] kg/m 2 in 2020 compared to 0 •43 [0 •39,0 •47] kg/m 2 in previous years). BMI adj accelerated most in ages 8-12 and least in ages 15-17.Interpretation: Children's rate of unhealthy weight gain increased notably during the COVID-19 pandemic across demographic groups, and most prominently in children already vulnerable to unhealthy weight gain. This data can inform policy decisions critical to child development and health as the pandemic continues to unfold.