In the process of achieving China’s “dual carbon” goals, it is urgent to encourage and guide residents to actively practice low-carbon behaviors. The influence mechanism of online social networks on residents’ low-carbon behavior and the mediating role of low-carbon cognition were examined. The results show that network scale and relationship strength, as two dimensions of measuring online social networks, have significant positive effects on the low-carbon behavior of residents in both private and public domains. Low-carbon cognition plays a mediating role between online social networks and low-carbon behaviors, indicating that online social networks significantly improve residents’ low-carbon behaviors in the private and public domains through multi-channel information extraction and key information capture. It is necessary to cultivate a well-functioning social communication space for the public, promote the frequency and relationship channels of residents’ online communication, and widely publicize and popularize low-carbon knowledge through social platforms.