Neoplatonism (also Neo-Platonism) is an ancient school of philosophy beginning in the 3rd century A.D. It was based on the teachings of Plato and Platonists; but it interpreted Plato in many new ways, such that Neoplatonism was quite different from what Plato taught, though not many Neoplatonists would admit the distinction.

Neoplatonism began with the philosopher Plotinus, though Plotinus claimed to have received his teachings from Ammonius Saccas, an illiterate dock-worker in Alexandria. Plotinus's writings were assembled by his student Porphyry into the Six Enneads.

Plotinus taught the existence of an indescribable One, which emanated the rest of the universe as a sequence of lesser beings. Later Neoplatonic philosophers, especially Iamblichus, added hundreds of intermediate gods and beings as emanations between the One and humanity; but Plotinus' system was much simpler in comparison.

Later neoplatonic philosophers included Porphyry, Proclus, Iamblichus and Hypatia of Alexandria.

Neoplatonism was frequently used as a philosophical foundation for paganism, and as a means of defending paganism against Christianity; but many Christians were also influenced by Neoplatonism. In Christian versions of Neoplatonism, the One is identified as God. Most important of these was Pseudo-Dionysius the Areopagite, whose work was very influential in the Middle Ages. Augustine of Hippo converted to Christianity under the influence of Plotinus, leading most scholars to label Augustine a frank Neoplatonist; although, they note that Augustine's subordination of philosophy to scripture leads to striking differences from the non-Christian philosophy. Some scholars have shown that Neoplatonism was also influenced by Christian theology, notably through the belief systems known as Gnosticism.

In the middle ages, Neoplatonist ideas influenced the thinking of Jewish Kabbalists, such as Isaac the Blind. The kabbalists modified Neoplatonism according to their own monotheistic leanings, however. A famous Jewish Neoplatonic philosopher from the early middle ages was Solomon ibn Gabirol.

Neoplatonism was revived in the Italian Renaissance by figures such as Marsilio Ficino.