OBJECTIVE: To examine the association between social isolation and mortality and incident diseases in middle-aged adults in urban and rural communities from high-income, middle-income and low-income countries. DESIGN: Population-based prospective observational study. SETTING: Urban and rural communities in 20 high income, middle income and low income. PARTICIPANTS: 119 894 community-dwelling middle-aged adults. MAIN OUTCOME MEASURES: Associations of social isolation with mortality, cardiovascular death, non-cardiovascular death and incident diseases. RESULTS: Social isolation was more common in middle-income and high-income countries compared with low-income countries, in urban areas than rural areas, in older individuals and among women, those with less education and the unemployed. It was more frequent among smokers and those with a poorer diet. Social isolation was associated with greater risk of mortality (HR of 1.26, 95% CI: 1.17 to 1.36), incident stroke (HR: 1.23, 95% CI: 1.07 to 1.40), cardiovascular disease (HR: 1.15, 95% CI: 1.05 to 1.25) and pneumonia (HR: 1.22, 95% CI: 1.09 to 1.37), but not cancer. The associations between social isolation and mortality were observed in populations in high-income, middle-income and low-income countries (HR (95% CI): 1.69 (1.32 to 2.17), 1.27 (1.15 to 1.40) and 1.47 (1.25 to 1.73), respectively, interaction p=0.02). The HR associated with social isolation was greater in men than women and in younger than older individuals. Mediation analyses for the association between social isolation and mortality showed that unhealthy behaviours and comorbidities may account for about one-fifth of the association. CONCLUSION: Social isolation is associated with increased risk of mortality in countries at different economic levels. The increasing share of older people in populations in many countries argues for targeted strategies to mitigate its adverse effects.