The United Nations' Sustainable Development Goal 16 (SDG 16) aims to promote peaceful, inclusive societies and build effective, accountable institutions. Despite the growing influence of online media on political activities in the digital age and the increasing importance of achieving SDG 16 through political participation, there has been a lack of empirical research on this topic. This cross-cultural comparative study investigates the relationships among online communication, social capital, and political participation in South Korea and the United States. Online surveys were conducted in both countries, focusing on people's interest and trust in online communication, types of online social capital (bonding and bridging), and engagement in conventional and unconventional forms of political participation. Our findings underscore the pivotal role of social media as a platform for citizens to voice their political opinions and partake in democratic processes. We discovered a significant correlation between heightened interest and trust in online communication and increased political participation in both countries. This correlation is instrumental in achieving SDG 16.7, which advocates for inclusive and participatory decision-making. This study also highlights the nuanced differences in the relationship between online social capital and political participation. In South Korea, bridging online social capital is associated with unconventional political activities, whereas in the United States, bonding online social capital is linked to these forms of engagement. However, in both countries, participation in presidential elections is associated with bridging online social capital, providing a deeper understanding of the complex dynamics at play. The potential of social media to expand opportunities for citizens to participate in the political process and hold their governments accountable contributes to achieving SDG 16.6, which aims to develop effective, accountable, and transparent institutions. However, the study also serves as a reminder of the challenges posed by online polarization and misinformation. To fully realize the benefits of social media in promoting inclusive and sustainable social development, addressing these issues and fostering healthy online communities are essential.In conclusion, this study underscores the importance of promoting online communication and building robust online communities as key strategies for achieving SDG 16, engaging a wide range of stakeholders in the process of building a more equitable and sustainable future.