White nationalists represent a growing security threat worldwide. Of concern are their narratives and conspiracy theories that position immigrants of diverse color and religion as the feared “other”—outgroups that are perceived to threaten the values, resources and safety of White people. We use survey data from 570 White Australians to better understand how and why such perceived “outgroup threats” elicit prejudice toward non-White immigrants. We find that perceived outgroup threats are strongly associated with greater prejudice. Importantly, we find that belief in The Great Replacement conspiracy theory explains the outgroup threat/prejudice relationship, while heightened feelings of uncertainty strengthen the relationship.