Ultimate reality is "the supreme, final, and fundamental power in all reality". It refers to the most fundamental fact about reality, especially when it is seen as also being the most valuable fact. This may overlap with the concept of the Absolute in certain philosophies.

Anaximander (c. 610 – c. 546 BCE) believed that the ultimate substance of the universe, generally known as arche, was apeiron, an infinite and eternal substance that is the origin of all things.

Aristotle (384–322 BCE) held that the unmoved mover "must be an immortal, unchanging being, ultimately responsible for all wholeness and orderliness in the sensible world" and that its existence is necessary to support everyday change.

Democritus (c. 460–370 BCE) and Epicureanism (c. 307 BCE) rejected the idea of ultimate reality, saying that only atoms and void exist, but they do have the eternal, unbounded, and self-caused nature of non-materialistic views of the concept.

In Neoplatonism (3rd century CE), the first principle of reality is "the One" which is a perfectly simple and ineffable principle which is the source of the universe, and exists without multiplicity and beyond being and non-being.

Stoic physics (c. 300 BCE – 3rd century CE) called the primitive substance of the universe pneuma or God, which is everything that exists and is a creative force that develops and shapes the cosmos.

In Theravada Buddhism, Nirvana is ultimate reality. Nirvana is described in negative terms; it is unconstructed and unconditioned.

Mahāyāna Buddhism has different conceptions of ultimate reality, which is framed within the context of the two truths, the relative truth of everyday things and the ultimate truth. Some traditions, specifically those who rely on the Madhyamaka philosophy, reject the notion of a truly existing or essential ultimate reality, regarding any existent as empty (sunyata) of inherent existence (svabhava). Modern philosophers like Jan Westerhoff interpret this a species of anti-foundationalism, or a metaphysical anti-realism.