Background: Sedentary behavior is associated with increased risk of chronic disease and sedentary behavior is increasing among adolescents. Data on changes in sedentary behavior in developing countries are limited. Purpose: To describe 5-year longitudinal changes in nonschool sedentary hours among urban adolescents in Ho Chi Minh City, and to identify correlates with this change. Methods: This is a 5-year longitudinal cohort with systematic random sampling of 759 students from 18 junior high schools. All measures were taken annually between 2004 and 2009. Sedentary behavior was assessed by self-report and accelerometry. Generalized linear latent and mixed models were used to analyze the data in 2011. Results: Between 2004 and 2009, self-reported time spent in nonschool sedentary behavior increased from 498 to 603 minutes/day. In the 5th survey year, boys and girls (aged 16 years) were, respectively, 3.6 times (95% CI=2.3, 6.0) and 3.1 times (95% CI= 1.8, 5.0) more likely to spend ≥2 hours/day on screen time compared with baseline (aged 12 years). Accelerometer data adjusted for wearing time revealed that boys and girls aged 16 years had, respectively, 78 minutes/day (95% CI=48, 104) and 69 minutes/day (95% CI=34, 95) more nonschool sedentary time than those at the first accelerometer assessment (at age 13 years). Girls in the highest socioeconomic quartile spent an additional 90 minutes/day in sedentary behavior compared with girls in the lowest quartile (95% CI=52, 128). Conclusions: Nonschool sedentary behavior increased among Vietnamese adolescents with age. The largest increase was in recreational screen time (28%), which would be the most obvious target for preventive health strategies. © 2013 American Journal of Preventive Medicine.