The Leyland Cypress, Cupressus × leylandii, is often referred to as just Leylandii. It is a fast-growing evergreen tree much used in horticulture, primarily for hedges and screens.

The Leyland Cypress is a hybrid between the Monterey Cypress, Cupressus macrocarpa, and the Nootka Cypress, Cupressus nootkatensis, family Cupressaceae. The hybrid has arisen on nearly 20 separate occasions, always by open pollination, showing the two species are readily compatible and closely related. Two other similar hybrids have also been raised, both involving Nootka Cypress with other Cupressus species:

Cupressus arizonica var. glabra × Cupressus nootkatensis (Cupressus × notabilis)

Cupressus lusitanica × Cupressus nootkatensis (Cupressus × ovensii)

The taxonomic status of Nootka Cypress is disputed; in the past, it was widely regarded as belonging in the genus Chamaecyparis, and in 2002 it was classified in a new genus Xanthocyparis. The most recent treatments (2004), however, transfer it to Callitropsis. In any of these treatments, the hybrids becomes very unusual in being intergeneric hybrids, the only ones ever reported among the Gymnosperms. In fact the very existence of these hybrids, and their ease of formation, is a further strong pointer (in addition to genetic and morphological evidence) for the treatment of Nootka cypress in Cupressus. It may be added that attempts to cross Nootka cypress with other Chamaecyparis species have been universally unsuccessful.

Where Nootka Cypress is treated in Chamaecyparis, the name of the hybrid becomes × Cupressocyparis leylandii, and where treated in Xanthocyparis, it becomes × Cuprocyparis leylandii.

Leyland Cypresses are commonly planted in gardens to provide a quick boundary or shelter hedge. However, their rapid growth, heavy shade and great potential height (often over 20 m tall in garden conditions, and they can reach at least 35 m) make them problematic. In Britain it has been the source of a number of high profile disputes between neighbours, even leading to violence (and in one recent case, murder), because of its capacity to cut out light.

It is poorly adapted to areas with hot summers, such as the southern half of the U.S.A., and in these areas becomes - perhaps fortunately - very prone to the disease cypress canker caused by the fungus Seridium cardinale. This causes extensive dieback and ultimately death of the tree. In California's Central Valley, they rarely live more than ten years before succumbing, and not much longer in southern states like Alabama. In these areas, the canker-resistant Arizona Cypress is much more successful.