Due to the pandemic of childhood obesity and thus obesity-related hypertension, improvements in treatment availability are needed. Hence, we investigated whether reductions in blood pressure (BP) would occur in children with overweight and obesity exhibiting prehypertension/hypertension during a community-based overweight and obesity treatment program, and if changes in body mass index (BMI) are associated with changes in BP. The study included 663 children aged 3-18 years with a BMI ⩾85th percentile for sex and age that entered treatment from June 2012 to January 2015. Height, weight and BP were measured upon entry and every 3-6 months. BMI and BP s.d. scores (SDSs) were calculated according to sex and age, or sex, age and height. Prehypertension was defined as a BP SDS ⩾1.28 and <1.65. Hypertension was defined as a BP SDS ⩾1.65. Upon entry, 52% exhibited prehypertension (11.9%) or exhibited hypertension (40.1%). After 12 months (range: 3-29) of treatment, 29.3% of the children with prehypertension/hypertension were normotensive. Children with systolic prehypertension/hypertension upon entry reduced their systolic BP SDSs by 0.31 (95% confidence interval (CI): 0.70-0.83, P<0.0001). Children with diastolic prehypertension/hypertension upon entry reduced their diastolic BP SDSs by 0.78 (95% CI: 0.78-0.86, P<0.0001). BMI SDS changes were positively associated with BP SDS changes (P<0.0001). Nonetheless, some children reduced BP SDSs while increasing their BMI SDSs, and prehypertension/hypertension developed in 23.3% of the normotensive children despite reductions in BMI SDSs (P<0.0001). These results suggest that community-based overweight and obesity treatment can reduce BP, and thus may help improve treatment availability.