Objective This study aims to investigate the prevalence and psychological impact of social isolation and loneliness in South Korea. Loneliness and social isolation have been regarded as a risk to both physical and mental health. However, most studies have focused on the elderly; hence, there are limited studies on the characteristics of socially isolated or lonely people considering age. Methods A sample of 1,700 participants was selected from three major cities in South Korea. In-person interviews were conducted to evaluate loneliness, social isolation and mental health status. Results Among the participants, the prevalence of social isolation and loneliness was 17.8% and 4.1%, respectively. Males decreased the odds of loneliness (AOR 0.49, 95% CI=0.28-0.87), while increasing the odds of social isolation (AOR 1.44, 95% CI=1.12-1.86) after adjusting for age and sex. Greater depressive and social phobic symptoms were associated with increased odds of loneliness and social isolation. Conclusion Social isolation and loneliness are prevalent among Koreans and associated with depression, social phobic symptoms, and suicidality. This study provides a foundation for further research to investigate nationwide prevalence and a more in-depth analysis of loneliness and social isolation.