Myrcene, or β-Myrcene, is an olefinic natural organic compound. It is classified as a hydrocarbon, more precisely as a monoterpene. Terpenes are dimers of isoprene, and myrcene is one of the most important. It is a component of the essential oil of several plants including bay, cannabis, ylang-ylang, wild thyme, parsley, and hops. It is produced mainly semi-synthetically from myrcia, from which it gets its name. It is a key intermediate in the production of several fragrances. α-Myrcene is the name for the structural isomer 2-methyl-6-methylene-1,7-octadiene, which is not found in nature and is little used.

Myrcene has an analgesic effect and is likely to be responsible for the medicinal properties of lemongrass tea. It has anti-inflammatory properties through Prostaglandin E2. The analgesic action can be blocked by naloxone or yohimbine in mice which suggests mediation by "α 2-Adrenoceptor stimulated release of endogenous opioids".

Very high doses of myrcene potentiate barbiturate induced sleep in mice, further supporting the suggestions of sedative activity. It is likely that this is responsible for the use of hops and lemongrass as sedatives.

Plants which contain myrcene are Cannabis, Hops, Houttuynia, Lemongrass, Mangoes, Myrcia, Thyme, Verbena, and West Indian Bay Tree.