The Atacama Pathfinder Experiment (APEX) is a radio telescope located at Llano de Chajnantor Observatory in the Atacama Desert of northern Chile. It is a single-dish modified ALMA prototype antenna.

APEX is operated by 3 European research institutes: The Max Planck Institut für Radioastronomie (MPIfR), Onsala Space Observatory (OSO), and the European Southern Observatory (ESO).