Based on the spiral of silence theory, this study derived motivational factors for SNS users' opinion expression regarding social discourse and examined the process by which individuals voice their opinions or remain silent on social media depending on the climate of public opinion. This study proposed perceived opinion support, fear of negative evaluation, self-censorship, and self-monitoring as motivational factors for opinion expression, and empirically examined the effects of self-censorship and self-monitoring on willingness to express opinions publicly. To test the research model, data were collected from adults nationwide who use SNS and analyzed using the PLS (Partial Least Squares) technique. The results showed that perceived opinion support had a negative effect on fear of negative evaluation, and fear of negative evaluation had a negative effect on willingness to express opinions publicly, thereby supporting the basic hypothesis of the spiral of silence theory. Additionally, fear of negative evaluation was found to have a positive effect on both self-censorship and self-monitoring; however, the paths from these two variables to willingness to express opinions publicly showed divergent results—the former exhibited a negative effect while the latter exhibited a positive effect. This study identified and addressed the theoretical limitations that have been overlooked in the spiral of silence theory and presented a new theoretical framework for understanding the public opinion formation process. Furthermore, the study applied this framework to individuals' opinion expression processes in the social media environment and empirically demonstrated the specific causal mechanisms involved.