Juvenile hormones (JHs) are a group of acyclic sesquiterpenoids that regulate many aspects of insect physiology. The first discovery of a JH was by Vincent Wigglesworth. JHs regulate development, reproduction, diapause, and polyphenisms. The chemical formula for juvenile hormone is ${\displaystyle {\ce {C_18H_30O_3}}}$.

In insects, JH (formerly neotenin) refers to a group of hormones, which ensure growth of the larva, while preventing metamorphosis. Because of their rigid exoskeleton, insects grow in their development by successively shedding their exoskeleton (a process known as molting).

Juvenile hormones are secreted by a pair of endocrine glands behind the brain called the corpora allata. JHs are also important for the production of eggs in female insects.

JH was isolated in 1965 by Karel Sláma and Carroll Williams and the first molecular structure of a JH was solved in 1967.

Most insect species contain only juvenile growth hormone (JH) III. To date JH 0, JH I, and JH II have been identified only in the Lepidoptera (butterflies and moths). The form JHB₃ (JH III bisepoxide) appears to be the most important JH in the Diptera, or flies. Certain species of crustaceans have been shown to produce and secrete methyl farnesoate, which is juvenile hormone III lacking the epoxide group. Methyl farnesoate is believed to play a role similar to that of JH in crustaceans.

Being a sesquiterpenoid, JH chemical structure differs significantly from the structure of other animal hormones. Some JH analogs have been found in conifers.