Several studies have explored the causes and consequences of public engagement with misinformation and, more recently, COVID-19 misinformation. However, there is still a need to understand the mechanisms that cause misinformation propagation on social media. In addition, evidence from non-Western societies remains rare. This study reports on survey evidence from eight countries to examine whether social media fatigue can influence users to believe misinformation, influencing their sharing intentions. Our insights also build on prior cognitive and personality literature by exploring how this mechanism is conditional upon users' cognitive ability and narcissism traits. The results suggest that social media fatigue can influence false beliefs of misinformation which translates into sharing on social media. We also find that those with high levels of cognitive ability are less likely to believe and share misinformation. However, those with low cognitive ability and high levels of narcissism are most likely to share misinformation on social media due to social media fatigue. This study is one of the first to provide cross-national comparative evidence highlighting the adverse effects of social media fatigue on misinformation propagation and establishing that the relationship is not universal but dependent on both cognitive and dark personality traits of individuals.