Tetrahydrocannabinol (pronounced /ˌtɛtrəˌhaɪdrɵkəˈnæbɨnɒl/ tet-rə-HYE-drə-kə-NAB-i-nol) (THC), also known as delta-9-tetrahydrocannabinol (Δ9-THC), Δ1-THC (using an older chemical nomenclature), or dronabinol, is the main psychoactive substance found in the Cannabis plant.

It was first isolated by Raphael Mechoulam, Yechiel Gaoni, and Habib Edery from the Weizmann Institute of Science in Rehovot, Israel in 1964.In pure form, it is a glassy solid when cold and becomes viscous and sticky if warmed. An aromatic terpenoid, THC has a very low solubility in water, but good solubility in most organic solvents.

Like most pharmacologically-active secondary metabolites of plants, THC in Cannabis is assumed to be involved in self-defense, perhaps against herbivores.THC also possesses high UV-B (280-315 nm) absorption properties, which, it has been speculated, could protect the plant from harmful UV radiation exposure.

Dronabinol is the International Nonproprietary Name (INN) for a pure isomer of THC, (-)-trans-Δ9-tetrahydrocannabinol, that is, the main isomer in Cannabis. It is sold as Marinol (a registered trademark of Solvay Pharmaceuticals). Dronabinol is also marketed, sold, and distributed by PAR Pharmaceutical Companies under the terms of a license and distribution agreement with SVC pharma LP, an affiliate of Rhodes Technologies.