Aim Health-related behaviors during adolescence lay the foundation of health status during later life. Most of the studies on health behaviors among adolescents in India have a focus on adolescent girls. India has 133 million adolescent boys; it is important to understand patterns and determinants of physical inactivity and poor dietary intake among them. Method We analyze levels and patterns of physical activity and dietary habits using data from the 'Understanding Lives of the Adolescents and Young Adults (UDAYA)' Survey, 2016, from Bihar, one of the less developed states in India. The analysis was carried out using data collected from 2004 adolescent boys aged 14-19 years covering 33,900 households. Multivariate logistic regression analysis was used to examine the effect of predictor variables on physical activity and dietary habits. Results Most adolescent boys (71%) engaged in physical activities during the one month before the survey. Fifteen percent did not engage in physical activity during the reference period and 13% never participated in such activities. One-fourth consumed three or more food groups and 6% did not daily consume any of the six selected food groups. Participants' age, place of residence, schooling status, and co-residence with siblings are strong predictors of physical activity. Place of residence, caste and economic status, household head's gender, schooling of household head, and number of siblings are factors identified as predictors of dietary habits. Conclusion This study underscores the urgent need for steps to enhance participation in physical activities and to promote daily consumption of meals with adequate dietary diversity among adolescent boys.