During the COVID-19 pandemic, governments use direct persuasion to encourage social isolation. Since self-persuasion is a more effective method of encouraging behavioural changes, using an experimental approach, we compared direct persuasion to self-persuasion on underlying motivations for voluntary social isolation during the COVID-19 pandemic. We asked the participants (N = 375) to write three arguments in support of social isolation (self-persuasion condition) or to evaluate three government graphics containing arguments for social isolation (direct persuasion condition). Then we asked the participants to evaluate perceived own vulnerability to COVID-19, the perceived severity of COVID-19, moral obligation to socially isolate and the attitude toward social isolation. Self-persuasion had a significant impact on the moral obligation to socially isolate, and through it on self-isolation intention. We also found evidence that individuals who perceived greater benefits from social isolation and who perceived a higher severity of COVID-19 have a higher intention to socially isolate. Significant sex and age differences also emerged. Our findings provide new insights into mechanisms of self-persuasion and underlying motivations that influence social isolation during the COVID-19 pandemic.