There is growing concern that the coronavirus disease 2019 (COVID-19) pandemic is exacerbating childhood obesity. We sought to examine the effects of the pandemic on weight and weight-related behaviours among children with overweight and obesity participating in an ongoing cluster randomized controlled trial of a paediatric practice-based weight intervention with 2 study arms: nutritionist-delivered coaching telephone calls over 8 weeks with an accompanying workbook on lifestyle changes versus the same workbook in eight mailings without nutritionist coaching calls. In a pooled, secondary analysis of 373 children in central Massachusetts (aged 8-12 years, 29% Latinx, 55% White, 8% Black), the monthly rate of BMI increase more than doubled for those children whose 6-month study visit occurred post-pandemic onset (n = 91) compared to children whose 6-month study visit occurred pre-pandemic onset (n = 282) (0.13 kg/m2 versus 0.05 kg/m2 ; ratio = 2.47, p = 0.02). The post-pandemic onset group also had a significant decrease in activity levels (β -8.18 MVPA minutes/day, p = 0.01). Caloric intake and screen time did not differ between the pre- and post-pandemic onset groups. These findings show that after the start of the pandemic, children with overweight and obesity experienced an increase in weight and decrease in activity levels. This data can inform public health strategies to address pandemic-related effects on childhood obesity.