Tea tree oil, also known as melaleuca oil, is an essential oil with a fresh camphoraceous odor and a colour that ranges from pale yellow to nearly colourless and clear. It is derived from the leaves of the tea tree, Melaleuca alternifolia, native to southeast Queensland and the northeast coast of New South Wales, Australia. The oil comprises many constituent chemicals and its composition changes if it is exposed to air and oxidizes. Commercial use of tea tree oil began in the 1920s, pioneered by the entrepreneur Arthur Penfold.

As a traditional medicine, it is typically used as a topical medication in low concentrations for the treatment of skin conditions, but there is little evidence of clinical efficacy.

Although tea tree oil is claimed to be useful for treating dandruff, acne, lice, herpes, insect bites, scabies, and skin fungal or bacterial infections, there is insufficient evidence to support any of these claims due to the limited quality of research. A 2015 Cochrane review of acne complementary therapies found a low-quality, single trial showing benefit on skin lesions compared to placebo.

According to the Committee on Herbal Medicinal Products (CHMP) of the European Medicines Agency, traditional usage suggests that tea tree oil is a possible treatment for "small superficial wounds, insect bites and small boils", and that it may reduce itching in minor cases of athlete's foot. The CHMP states that tea tree oil products should not be used on people under 12 years of age.

Tea tree oil is not recommended for treating nail fungus because it is not effective. It is not recommended for treating head lice in children because its effectiveness and safety have not been established and it could cause skin irritation or allergic reactions. There is no good evidence tea tree oil is an effective treatment for demodex mite infestations.