This study aimed to evaluate the relationships between sleep duration and overweight/obesity, obesity, and metabolic syndrome (MetS) and its components in children and adolescents. A total of 6,048 participants aged 10-18 years were divided into the following four sleep-duration groups based on age-specific sleep duration: i) very short; ii) short; iii) recommended; and iv) long. The participants in the very short sleep-duration group had an increased odds ratio (OR) of 1.76 for overweight/obesity, 1.69 for obesity, and 1.49 for elevated waist circumference (WC) compared with the recommended sleep-duration group. The subjects in the long sleep-duration group had an increased OR of 2.71 for elevated triglyceride (TG) compared with those in the recommended sleep-duration group. In subgroup analyses, boys in the very short sleep-duration group exhibited an increased OR of 1.78 for overweight/obesity compared with those in the recommended sleep-duration group. Compared with girls in the recommended sleep-duration group, those in the very short sleep-duration group exhibited an increased OR of 1.69 for overweight/obesity, 2.28 for obesity, and 1.57 for elevated WC; in contrast, girls in the very short sleep-duration group exhibited a decreased OR of 0.58 for elevated TG. The girls in the long sleep-duration group had an increased OR of 3.86 for elevated TG compared with those with recommended sleep-duration. Our results suggest that shorter sleep durations may be related to overweight/obesity, obesity, and central obesity, and longer sleep durations may be associated with elevated TG. However, the nature of these relationships may be dependent on sex.