α-Pinene is a clear colorless liquid with a turpentine odor. Less dense than water and insoluble in water. Vapors are heavier than air. Used as a solvent. α-Pinene is an organic compound of the terpene class, one of two isomers of pinene. It is an alkene and it contains a reactive four-membered ring. It is found in the oils of many species of many coniferous trees, notably the pine. It is also found in the essential oil of rosemary (Rosmarinus officinalis). Both enantiomers are known in nature; (1S,5S)- or (−)-α-Pinene is more common in European Pines, whereas the (1R,5R)- or ±α-isomer is more common in North America. The racemic mixture is present in some oils such as eucalyptus oil and orange peel oil.

At low exposure levels, α-Pinene is a bronchodilator in humans, and is highly bioavailable with 60% human pulmonary uptake with rapid metabolism or redistribution. α-Pinene is an anti-inflammatory via PGE1, and seems to be a broad-spectrum antibiotic. It exhibits activity as an acetylcholinesterase inhibitor, aiding memory. α-Pinene forms the biosynthetic base for CB2 ligands, such as HU-308.