In theology, the doctrine of divine simplicity says that God is simple (without parts). The general idea can be stated in this way: The being of God is identical to the "attributes" of God. Characteristics such as omnipresence, goodness, truth, eternity, etc., are identical to God's being, not qualities that make up that being as a collection, nor abstract entities inhering in God as in a substance; in other words, one can say that in God both essence and existence are one and the same. This is not to say that God is a simpleton or "simple" to understand. As Peter Weigel states, "Divine simplicity is central to the classical Western concept of God. Simplicity denies any physical or metaphysical composition in the divine being. This means God is the divine nature itself and has no accidents (properties that are not necessary) accruing to his nature. There are no real divisions or distinctions in this nature. Thus, the entirety of God is whatever is attributed to him. Divine simplicity is the hallmark of God’s utter transcendence of all else, ensuring the divine nature to be beyond the reach of ordinary categories and distinctions, or at least their ordinary application. Simplicity in this way confers a unique ontological status that many philosophers find highly peculiar." So when it comes to God's essential nature/attributes, there are no parts or accidents; this is not to be confused with, for example, God's accidental/contingent relation to the world (i.e. God's non-essential/contingent properties – not God's nature).

Varieties of the doctrine may be found in Jewish, Christian, and Muslim philosophical theologians, especially during the height of scholasticism, although the doctrine's origins may be traced back to ancient Greek thought, finding apotheosis in Plotinus' Enneads as the Simplex.