During the COVID-19 pandemic, as countries implemented regulations to curb the spread of COVID-19, social media users called on others to follow the measures while capturing and uploading images of those violating the rules. Guided by the frameworks, theory of planned behavior (TPB) and belief in a just world (BJW), the current study focuses on the context of Singapore to identify factors behind engaging in such acts of online shaming, and whether these constitute online vigilantism. Based on an online survey in Singapore (n = 1000), and using a binary logistic regression, this study found that those who perceive online shaming as a commonplace behavior, and who think it is expected of them to do so, were more likely to engage in online shaming. Conversely, attitude and perceived behavioral outcome did not predict online shaming. Additionally, those who have strong BJW for others were more likely to engage in online shaming, providing evidence that for some, online shaming is a form of online vigilantism. However, those with a strong BJW for self are less likely to engage in online shaming. These suggest that for some, online shaming is a form of online vigilantism, motivated by a sense of justice.