BACKGROUND: The 2020 World Health Organization Guidelines on Physical Activity and Sedentary Behavior are the first to provide evidence-based recommendations for reducing both dimensions of languorous behavior. The relationship between sitting time and exercise remains understudied among diverse adolescent populations worldwide. METHODS: The 2017 Sierra Leone Global School-based Student Health Survey was a nationally representative cross-sectional study of secondary school students. RESULTS: Of the 2798 participants, 82% did not engage in moderate or vigorous physical activity for at least an hour every day, 25% sat for 3 or more hours each day outside of school and homework, and 87% were physically inactive and/or sedentary based on those thresholds. Girls who never or rarely exercised tended to maintain light physical activity outside of school rather than sitting, while boys who exercised daily tended to be sedentary when they were not playing sports. IMPLICATIONS FOR SCHOOL HEALTH POLICY, PRACTICE, AND EQUITY: Students spend their waking hours doing a mix of sitting, light physical activity, and moderate or vigorous physical activity. Schools can help reduce sedentarism and increase movement among students. CONCLUSIONS: Home, school, and community health interventions may be useful for increasing energy expenditure among adolescents in low- and middle-income countries.