In the philosophy of religion, Reformed epistemology is a school of philosophical thought concerning the nature of knowledge (epistemology) as it applies to religious beliefs. The central proposition of Reformed epistemology is that beliefs can be justified by more than evidence alone, contrary to the positions of evidentialism, which argues that while non-evidential belief may be beneficial, it violates some epistemic duty. Central to Reformed epistemology is the proposition that belief in God may be "properly basic" and not need to be inferred from other truths to be rationally warranted. William Lane Craig describes Reformed epistemology as "One of the most significant developments in contemporary religious epistemology ... which directly assaults the evidentialist construal of rationality."

Reformed epistemology was so named because it represents a continuation of the 16th-century Reformed theology of John Calvin, who postulated a sensus divinitatis, an innate divine awareness of God's presence. More recent influences on Reformed epistemology are found in philosopher Nicholas Wolterstorff's Reason within the Bounds of Religion, published in 1976, and Alvin Plantinga's "Reason and Belief in God", published in 1983.

Although Plantinga's Reformed epistemology developed over three decades, it was not fully articulated until 1993 with the publication of two books in an eventual trilogy: Warrant: The Current Debate, and Warrant and Proper Function. The third in the series was Warranted Christian Belief, published in 2000. Other prominent defenders of Reformed epistemology include William Lane Craig, William Alston, Michael C. Rea, and Michael Bergmann.

The argument from a proper basis is an ontological argument for the existence of God related to fideism. Alvin Plantinga argued that belief in God is a properly basic belief, and so no basis for belief in God is necessary.

Alvin Plantinga is the best-known defender of reformed epistemology. Reformed epistemology includes two arguments against classical foundationalism. The first grew out of his earlier argument in God and Other Minds (1967). In that work Plantinga argued that if our belief in other minds is rational without propositional or physical evidence, then belief in God is also rational. In his 1993 works, Plantinga argued that according to classical foundationalism most of us are irrational for having many beliefs we cannot justify, but which foundationalism does not accept as properly basic. Plantinga's second argument against classical foundationalism is that it is self-referentially incoherent. It fails the test of its own rules, which require that it be either self-evident, incorrigible, or evident to the senses. William Lane Craig clarifies that properly basic beliefs can be "memory beliefs" (e.g., “I left the car keys in the dresser”) and "perceptual beliefs" (e.g., “I see a cat in the backyard”), but they can be defeasible, "that is to say, they can be mistaken".

In Plantinga's view, warrant is defined as the property of beliefs that makes them knowledge. Plantinga argues that a properly basic belief in God is warranted when produced by a sound mind, in an environment supportive of proper thought in accord with a design plan successfully aimed at truth. Because there is an epistemically possible model according to which theistic belief is properly basic and designed to form true belief in God, belief in God is probably warranted if theism is true. Plantinga does not argue that this model is true, but only that if it is true, theistic belief is also likely true, because then theistic belief would result from our belief-forming faculties functioning as they were designed.

This connection between the truth value of theism and its positive epistemic status suggests to some that the goal of showing theistic belief to be externally warranted requires reasons for supposing that theism is true (Sudduth, 2000). This point is answered by many theistic arguments which purport to provide sufficient propositional and physical evidence to warrant that belief, apart from reformed epistemology.

According to Reformed epistemology, belief in God can be rational and justified even without arguments or evidence for the existence of God. More specifically, Plantinga argues that belief in God is properly basic, and due to a religious externalist epistemology, he claims belief in God could be justified independently of evidence. His externalist epistemology, called "Proper functionalism", is a form of epistemological reliabilism.