Objectives To develop a Loneliness and Social Isolation scale (LSIS) that can measure both social isolation and loneliness in order to understand the degree of social isolation in Korea. Exploratory and confirmatory factor analysis was used to examine the factorial validity of the scale. Methods The subjects of the study were 300 adults aged 19 or older who visited Samsung Medical Center and voluntarily expressed their willingness to participate in this research. Exploratory factor analysis (n=150) and confirmatory factor analysis (n=150) were conducted to construct the factorial structure model and to determine the model fit. Results Exploratory factor analysis showed a three-factor structure with a total variance of 65.8%; factor 1 consisted of social support, factor 2 of social networks, and factor 3 of items representing loneliness. After conducting confirmatory factor analysis on the three-factor models, a three-factor model consisting of 8 items (LSIS-8) and a three-factor model consisting of 6 items (LSIS-6) showed significant goodness-of-fit. Internal consistency for all items was good (Cronbach’s α=0.774), and correlations with existing social isolation and loneliness measures were significant. Conclusion This study is meaningful as provides a tool that comprehensively measures social support, social networks, and loneliness. We believe that the application of such tools that are relatively easy to apply in communities will aid understanding of the current state of social isolation and loneliness in Korea.