The Heart Sutra (Sanskrit: Prajnaparamitahrdaya ) is a popular sutra in Mahayana Buddhism. Its Sanskrit title, Prajnaparamitahrdaya, can be translated as "The Heart of the Perfection of Wisdom".

The sutra famously states, "Form is empty" (sunyata). It is a condensed expose on the Buddhist Mahayana teaching of the Two Truths doctrine, which says that ultimately all phenomena are sunyata, empty of an unchanging essence. This emptiness is a characteristic of all phenomena, and not a transcendent reality, but also "empty" of an essence of its own. Specifically, it is a response to Sarvastivada teachings that "phenomena" or its constituents are real.