The World Health Organization set a target of a 15% relative reduction in the prevalence of insufficient physical activity (IPA) by 2025 among adolescents and adults globally. In Bangladesh, there are no national estimates of the prevalence of IPA among adolescents. The aim of this study was to estimate the prevalence of and risk factors associated with IPA among adolescent girls and boys. Data for 4865 adolescent girls and 4907 adolescent boys, collected as a part of a National Nutrition Surveillance in 2018–19, were analysed for this study. A modified version of the Global Physical Activity Questionnaire (GPAQ) was used to collect physical activity data. The World Health Organization recommended cut-off points were used to estimate the prevalence of IPA. Bivariate and multivariable logistic regression was performed to identify factors associated with IPA. Prevalences of IPA among adolescent girls and boys were 50.3% and 29.0%, respectively, and the prevalence was significantly higher among early adolescents (10–14 years) than late adolescents (15–19 years) among both boys and girls. The IPA prevalence was highest among adolescents living in non-slum urban areas (girls: 77.7%; boys: 64.1%). For both boys and girls, younger age, non-slum urban residence, higher paternal education and increased television viewing time were significantly associated with IPA. Additionally, residing in slums was significantly associated with IPA only among the boys. Higher maternal education was associated with IPA only among the girls. This study identified several modifiable risk factors associated with IPA among adolescent boys and girls in Bangladesh. These factors should be addressed through comprehensive public health interventions to promote physical activity among adolescent girls and boys.