Eucalyptus longissima is a species of mallee or small tree that is endemic to the south-west of Western Australia.

It has rough, fibrous or stringy bark on the trunk, smooth greyish brown bark above, glossy green, lance-shaped adult leaves, flower buds in group of between seven and thirteen, white flowers and shortened spherical fruit.

Eucalyptus longissima is a mallee that typically grows to a height of 8 m (26 ft) and forms a lignotuber.

[4] The specific epithet (longissima) is the superlative form of the Latin word longus, hence "longest", referring to the juvenile leaves, peduncles and pedicels.

[5] This eucalypt grows in sand or loam over limestone in the Coolgardie, Great Victoria Desert, Murchison and Yalgoo biogeographic regions of Western Australia.