The William Herschel Telescope (WHT) is an optical/near-infrared reflecting telescope which was built at the Observatorio del Roque de los Muchachos in the 1980s and saw first light on 1 June 1987. With a mirror width of 4.2m, it was the third largest single optical telescope in the world at the time of its construction and was the final telescope constructed by Grubb Parsons in their 150-year history. In La Palma, it forms part of the Isaac Newton Group of Telescopes, along with the INT and the Jacobus Kapteyn Telescope, all of which were under the control of the RGO until its closure in 1998.

The WHT was first conceived of in 1969 by the Northern Hemisphere Observatory Project, when the British astronomical community saw the need for telescopes of comparable power to the Anglo-Australian Telescope (AAT) in the northern hemisphere. The project, which was led by the RGO, originally envisaged a 4.5m telescope to accompany the INT and JKT. By 1979, to reduce costs, a high performance, more compact telescope was chosen with a reduced aperture and focal length, allowing for a smaller dome. Grubb Parsons began work on the WHT in their workshop at Newcastle-upon-Tyne in 1983, and the telescope was shipped to La Palma in 1985.

Equipped with a wide range of instrumentation, the WHT has been used to conduct a range of research, including the first identification of a supermassive black hole at the centre of the Milky Way, and the first optical observation of a gamma-ray burst.