The Extremely Large Telescope (ELT) is an astronomical observatory currently under construction. When completed, it will be the world's largest optical/near-infrared extremely large telescope. Part of the European Southern Observatory (ESO) agency, it is located on top of Cerro Armazones in the Atacama Desert of northern Chile.

The design consists of a reflecting telescope with a 39.3-metre-diameter (130-foot) segmented primary mirror and a 4.2 m (14 ft) diameter secondary mirror, and will be supported by adaptive optics, eight laser guide star units and multiple large science instruments. The observatory's design will gather 100 million times more light than the human eye, equivalent to about 10 times more light than the largest optical telescopes existing as of 2023, and will correct for atmospheric distortion. It has around 250 times the light gathering area of the Hubble Space Telescope and, according to the ELT's specifications, would provide images 16 times sharper than those from Hubble.

The project was originally called the European Extremely Large Telescope (E-ELT), but the name was shortened in 2017. The ELT is intended to advance astrophysical knowledge by enabling detailed studies of planets around other stars, the first galaxies in the Universe, supermassive black holes, and the nature of the Universe's dark sector, and to detect water and organic molecules in protoplanetary disks around other stars. As planned in 2011, the facility was expected to take 11 years to construct, from 2014 to 2025.

On 11 June 2012, the ESO Council approved the ELT programme's plans to begin civil works at the telescope site, with construction of the telescope itself pending final agreement with governments of some member states. Construction work on the ELT site started in June 2014. By December 2014, ESO had secured over 90% of the total funding and authorized construction of the telescope to start, which will cost around one billion euros for the first construction phase. The first stone of the telescope was ceremonially laid on 26 May 2017, initiating the construction of the dome's main structure and telescope. As of 2023, first light is planned for 2028.