Religious fundamentalism remains a significant force in global politics and religion. Despite a range of problems arising from fundamentalism, the beliefs fundamentalists hold can seem quite reasonable. This paper considers whether, in fact, fundamentalist beliefs are rational by drawing on recent ideas in contemporary epistemology. The paper presents a general theory of fundamentalist beliefs in terms of their propositional content and the high credence levels attributed to them. It then explores the way these beliefs are both acquired and retained by applying ideas from the social epistemology of echo chambers and group belief. The paper then considers three accounts of the rationality of belief: evidentialist, reliabilist, and virtue-theoretic. It is argued that fundamentalist beliefs can be reasonable on evidentialist standards, but are nevertheless still problematic on reliabilist and virtue standards, since they are formed in environments that are not truth-conducive and which cultivate intellectual vice.