A dark fibre or unlit fibre is an unused optical fibre, available for use in fibre-optic communication. Dark fibre may be leased from a network service provider.

Dark fibre originally referred to the potential network capacity of telecommunication infrastructure. Because the marginal cost of installing additional fibre optic cables is very low once a trench has been dug or conduit laid, a great excess of fibre was installed in the US during the telecom boom of the late 1990s and early 2000s. This excess capacity was later referred to as dark fibre following the dot-com crash of the early 2000s that briefly reduced demand for high-speed data transmission.

These unused fibre optic cables later created a new market for unique private services that could not be accommodated on lit fibre cables (i.e., cables used in traditional long-distance communication).