Purpose: The purpose of this study was to objectively assess levels of physical activity (PA) and sedentary behavior (SB) of Chinese city children and youth aged 9 to 17 years old using accelerometers and to examine their differences by gender, age, grade, and weight status. Method: The PA and SB of 2,163 students in 4th grade through 11th grade ("M"[subscript age] = 160.87 ± 27.00 months [13.41 ± 2.25 years], 50.21% boys) from 11 cities in China were measured by accelerometers. The amount of time spent in SB, light PA, moderate PA, vigorous PA, and moderate-to-vigorous PA (MVPA) was computed based on cutoff points developed specifically for the Chinese children and youth. The participants were classified into normal-weight, overweight, and obese groups based on their body mass index (BMI). Paired-sample "t" tests were conducted to examine the differences in PA and SB between weekdays and weekend days. Multivariate analysis of variance was used to test the differences in PA and SB variables by gender, age, grade, and weight status, respectively. Results: Chinese city children and youth spent an average of 28.26 ± 17.66 min/day in MVPA and 521.50 ± 110.02 min/day in SB. Only 9.4% of boys and 1.9% of girls met the recommendation of 60 min/day of MVPA. Chinese city children and youth were more active during weekdays than during weekend days, and boys were more active than girls; older children and youth spent more daily time in MVPA, but also spent more time being sedentary. No differences in PA and SB were found across different BMI categories. Conclusion: The findings warn of the insufficiency of PA and the excess of SB in Chinese city children and youth.