Presuppositional apologetics, shortened to presuppositionalism, is an epistemological school of Christian apologetics that examines the presuppositions on which worldviews are based, and invites comparison and contrast between the results of those presuppositions.

It claims that apart from presuppositions, one could not make sense of any human experience, and there can be no set of neutral assumptions from which to reason with a non-Christian. Presuppositionalists claim that Christians cannot consistently declare their belief in the necessary existence of the God of the Bible and simultaneously argue on the basis of a different set of assumptions that God may not exist and Biblical revelation may not be true.^{[failed verification]} Two schools of presuppositionalism exist, based on the different teachings of Cornelius Van Til and Gordon Haddon Clark. Presuppositionalism contrasts with classical apologetics and evidential apologetics.

Presuppositionalists compare their presupposition against other ultimate standards such as reason, empirical experience, and subjective feeling, claiming presupposition in this context is:

a belief that takes precedence over another and therefore serves as a criterion for another. An ultimate presupposition is a belief over which no other takes precedence. For a Christian, the content of scripture must serve as his ultimate presupposition… This doctrine is merely the outworking of the 'lordship of the Christian God' in the area of human thought. It merely applies the doctrine of scriptural infallibility to the realm of knowing.

Presuppositionalists contrast their approach with the other schools of Christian apologetics by describing the others as assuming that the world is intelligible apart from belief in the existence of God. Then they argue on purportedly neutral grounds to support trusting the Christian scriptures and the existence of God. Specifically, presuppositionalists describe Thomistic (also "traditional" or "classical") apologetics as concentrating on the first aspect of apologetics with its logical proofs for the existence of God. Thus assuming common ground with the non-Christian and using a piece-by-piece methodology. In this scheme, the common foundation of neutral brute facts leads to a generic concept of deity, then to the various characteristics of the Christian God as revealed in Scripture, and so forth. Piece-by-piece, Christian theology is built up from a neutral common ground.

Presuppositionalists assert that many of the classical arguments are logically fallacious, or do not prove enough, when used as arguments to prove the existence or character of God. They criticize both the assumption of neutrality and the "block house" or "piecemeal" method for failing to start at the level of the controlling beliefs of worldviews and implicitly allowing non-Christian assumptions from the start, thereby trying to build a Christian "house" on a non-Christian "foundation". Evidentialists demur from this assessment, claiming that presuppositionalism amounts to fideism because it rejects the idea of shared points of reference between the Christian and non-Christian from which they may reason in common.

The conclusion of evidential apologetics is that the Bible's historical accounts and other truth-claims are more probably true than false, thus the whole of scriptural revelation may be rationally accepted, and where we can't approach absolute certainty we must accept the explanations most likely to be true. The goal of presuppositional apologetics, on the other hand, is to argue that the assumptions and actions of non-Christians require them to believe certain things about God, man, and the world which they claim not to believe. This type of argument is technically called a reductio ad absurdum in that it attempts to reduce the opposition to holding an absurd, i.e., self-contradictory position; in this case, both believing in facts of Christian revelation (in practice) and denying them (in word). So, in essence, evidential apologetics attempts to build upon a shared acceptance of self-evident or worldview-neutral facts, while presuppositional apologetics attempts to claim all facts for the Christian worldview as the only framework in which they are intelligible.

Another way presuppositionalism has been developed is by first presupposing reason as the laws of thought (common to all thinkers).This critically examines beliefs for meaning, and finally constructing a coherent worldview from the ground up. This way of arguing has been called Rational Presuppositionalism. They postulate that thinking (or reasoning) is presuppositional in that we think of the less basic things, like the nature of man, in light of the more basic things, the existence or non-existence of God. How we view mankind's purpose and destiny depends on our assumption about man's origin. Philosopher Surrendra Gangadean (1943–2022) opened his book Philosophical Foundation: A Critical Analysis of Basic Beliefs by stating the following. “Some things are clear. The basic things are clear. The basic things about God and man and good and evil are clear to reason.” He argues step by step from knowledge is possible to the existence of God to the Good for man as knowledge of the eternal Creator.