This online experiment used the Theory of Planned Behavior to examine the impact of exposure to conspiracy social media messages on Chinese young adults' perceptions of the HPV vaccine. Three major findings were identified. First, exposure to anti-vaccine conspiracy theories resulted in less favorable attitudes toward the HPV vaccine, less positive perceived norms regarding getting vaccinated, and weaker vaccination intentions. Second, people who were more knowledgeable about the HPV vaccine had more favorable attitudes, more positive norms, higher perceived behavioral control, and greater behavioral intentions. Finally, vaccine knowledge moderated the effect of exposure to anti-vaccine conspiracy theories. Our results suggest that, among Chinese young adult social media users, preexisting knowledge of the HPV vaccine may have a protective effect against exposure to online conspiracy theories.