An ontological argument for the existence of God (or simply ontological argument) is any one of a category of arguments for the existence of God. The exact criteria for the classification of ontological arguments are not widely agreed, but the arguments typically start with the definition of God and conclude with his necessary existence, using mostly or only a priori reasoning and little reference to empirical observation.

It is widely accepted that the first ontological argument was proposed by Anselm of Canterbury. Anselm defined God as the greatest possible being we can conceive and argued that this being could exist in the mind. He suggested that, if the greatest possible being exists in the mind, it must also exist in reality. If it only exists in the mind, a greater being is possible—one which exists in the mind and in reality. Seventeenth century French philosopher René Descartes deployed a similar argument. Descartes published several variations of his argument, each of which centered on the idea that God's existence is immediately inferable from any "clear and distinct" idea of a supremely perfect being. In the early eighteenth century, Gottfried Leibniz augmented Descartes' ideas in an attempt to prove that a "supremely perfect" being is a coherent concept. A more recent ontological argument came from Kurt Gödel, who proposed a formal argument for God's existence. Norman Malcolm revived the ontological argument in 1960 when he located a second, stronger ontological argument in Anselm's work; Alvin Plantinga challenged this argument and proposed an alternative, based on modal logic. Attempts have also been made to validate Anselm's proof using an automated theorem prover. Other arguments have been categorised as ontological, including those made by Islamic philosopher Mulla Sadra.