<p>Sharing of fake news and conspiracy beliefs are prevalent concerns in online spaces, especially in contexts that involve politicized topics and identities. However, it is less clear to what extent social media platforms and users partisan identities are associated with such behavior and beliefs, especially when comparing traditional and non-traditional partisan groups. Using a cross-sectional survey of Alt-Right supporters and non-Alt-Right Democrats and Republicans in the U.S., we investigate how political identity and use of mainstream and dark social media platforms is related to fake news sharing behaviors and beliefs in two conspiracies (QAnon, 5 G causes COVID-19). We find that both mainstream and dark social media use are associated with more fake news sharing and conspiracy beliefs among those with Alt-Right beliefs compared to non-Alt-Right groups. This study adds to our understanding of how user characteristics and platform dynamics can separately and in tandem influence online behaviors in democratic societies.</p>