The Atacama Pathfinder Experiment (APEX) is a radio telescope located at 5,100 meters above sea level, at the Llano de Chajnantor Observatory in the Atacama desert, in northern Chile, 50 kilometers to the east of San Pedro de Atacama. The main dish has a diameter of 12 meters and consists of 264 aluminium panels with an average surface accuracy of 17 micrometres (r.m.s.). The telescope was officially inaugurated on September 25, 2005.

The APEX telescope is a modified ALMA (Atacama Large Millimeter Array) prototype antenna and is located at the site of the ALMA observatory. APEX is designed to work at sub-millimetre wavelengths, in the 0.2 to 2.0 mm range — between infrared light and radio waves, and to find targets that which ALMA will be able to study in greater detail. Submillimetre astronomy provides a window into the cold, dusty and distant Universe, but the faint signals from space are heavily absorbed by water vapour in the Earth's atmosphere. Chajnantor was chosen as the location for such a telescope because the region is one of the driest on the planet and is more than 750 m higher than the observatories on Mauna Kea, and 2400 m higher than the Very Large Telescope (VLT) on Cerro Paranal.

APEX is a collaboration between the Max Planck Institute for Radio Astronomy (MPIfR) at 50%, Onsala Space Observatory (OSO) at 23%, and the European Organisation for Astronomical Research in the Southern Hemisphere (ESO) at 27%. The telescope was designed and constructed by VERTEX Antennentechnik GmbH (Germany), under contract by MPIfR. The operation of APEX in Chajnantor is entrusted to ESO.

Due to the extreme altitude of the APEX telescope location, astronomers are hosted at the APEX base camp in Sequitor in the beautiful village of San Pedro de Atacama.