Online misinformation poses a significant threat to global challenges, such as the COVID-19 pandemic and climate change. Misinformation is disproportionality shared by people with extreme political attitudes, especially among the far right. To understand the psychological and neurocognitive processes that underlie misinformation sharing among extremists, we conducted a pre-registered, cross-cultural experiment with conservatives and far-right supporters in the US and Spain (N = 1,609) and a neuroimaging study with far-right supporters in Spain (N = 36). Individuals who felt their personal identity was fused with their political group were more likely to share misinformation, especially when the misinformation was related to issues that involve sacred moral values (e.g., immigration and nationalism). Analytical thinking was unrelated to misinformation sharing when the misinformation involved sacred values (vs. non-sacred values) and fact-checks had little or no effect in this sample, especially among hyper-partisans. Far-right supporters also showed increased activity in brain regions associated with theory of mind in response to posts with sacred values, highlighting the social dimension of misinformation sharing. These results suggest that political devotion plays a key role in misinformation sharing and that identity-based interventions may help curb misinformation for specific groups.