柏拉图主义（英语：Platonism），意指由古希腊哲学家柏拉图提出的哲学理论，或是被认为是始于柏拉图学说的哲学系统。广义的柏拉图主义（英语：platonism，通常用小写'p'来表示），是指认同理念形式是存在的、永恒的，并比世界中的现象更实在、更完美，甚至是唯一真正实在和完美的实体。这个体系还包括认为理念形式只能由灵魂所认识等，这个体系不一定接受柏拉图的所有见解，是与唯名论相反的一种哲学主张。

美国传统辞典解释为：The philosophy of Plato, especially insofar as it asserts ideal forms as an absolute and eternal reality of which the phenomena of the world are an imperfect and transitory reflection.

Platonism, rendered as a proper noun, is the philosophy of Plato or the name of other philosophical systems considered closely derived from it. In narrower usage, platonism, rendered as a common noun (with a lower case 'p', subject to sentence case), refers to the philosophy that affirms the existence of abstract objects, which are asserted to "exist" in a "third realm" distinct both from the sensible external world and from the internal world of consciousness, and is the opposite of nominalism (with a lower case "n"). Lower case "platonists" need not accept any of the doctrines of Plato.