Octadecaborane is an inorganic compound, a boron hydride cluster with chemical formula B18H22.

It is a colorless flammable solid, like many higher boron hydrides.

Although the compound has no practical applications, its structure is of theoretical and pedagogical interest.

The clusters are also of interest because the boron centers shared between the two subunits have an unusually high number of B-B interactions.

The isomers consists of two B9H11 polyhedral subunits, each having a decaborane-like form, joined at a B–B edge.