BACKGROUND/OBJECTIVES: Overweight and obesity among young people is a major concern with implications for social wellbeing. We aimed to evaluate the associations of physical activity (PA) and sedentary behaviour (SB) with loneliness among adolescents with overweight/obesity. METHODS: Data were used from the Global School-based Student Health Surveys, collected from 23 low-and-middle-income countries. RESULTS: Of the 12,133 participants aged 12-17 years with overweight/obesity (48.4% girls), 12% were lonely, with a wide variation across countries ranging from 4% in Laos to 32% in Afghanistan. Overall, 31% had high SB (≥3 hours/day) and 86% were not sufficiently active (<7 days/week of ≥ 60 mins/day). High SB and insufficient PA were positively associated with loneliness (aOR 1.22, 95% CI 1.08-1.38, aOR 1.37, 95% CI 1.18-1.59, respectively). CONCLUSIONS: Future research could examine the direction of these relationships and evaluate strategies to optimise these indicators of social wellbeing among adolescents with overweight/obesity.