This study examines the moderating effects of social media use on regionalist voting behavior in South Korea. Analyzing the survey data conducted during the 2017 Korean presidential election, we test how social media functions in electoral processes, particularly with respect to region-based voting in the Korean electorate. The findings of this study reveal that social media use affects region-based voting behavior among the Korean electorate by connecting people with different regional backgrounds in online political communication. That is, social media use can create “bridging” social capital rather than “bonding” social capital in society. In this respect, results differ significantly from findings in the 2012 presidential election. In 2012, only the independent effects of social media existed with a liberal bias, without revealing interaction with regional dummies. These independent effects disappeared in 2017, and different kinds of social media were statistically significant only when they functioned as moderating variables for regional dummies. This implies that as the functions of social media in the Korean election process have evolved in more complexity, they now are able to affect progressive as well as conservative voters.