An extreme precipitation event took place during the second half of January 2020 in the Atacama Desert. From Tacna, Peru to Iquique, Chile (18-21ºS) rainfall extended for several days producing floods, major damage to infrastructure, and affecting population in one of the driest deserts of the world. Analysis of surface-weather stations and reanalysis suggests that the most intense precipitation occurred in the Precordillera (2000 to 3600 m.a.s.l) on the western foothills of the Andes. The analysis based on surface observations, upper-air sounding, reanalysis and satellite data, suggests that at least four major factors were present to produce record-breaking precipitation: (i) a low-level circulation off-shore the Atacama Desert, potentially generated by the southward displacement of the Bolivian High and a trough located to the west over subtropical southeast Pacific, (ii) humidity advection via an atmospheric river-like structure, trapped along the coast ahead of the low-level cyclonic circulation, leading to increases in precipitable water vapor over the Atacama Desert, (iii) above-normal sea surface temperatures favoring moist conditions in the boundary layer and (iv) a strengthened local circulation, with low-level eastward moisture advection leading to forced orographic ascent along the Precordillera. These factors triggered thunderstorm development and precipitation mostly in the Precordillera, but also in the Lowlands, Pampas and Altiplano regions. Analysis of days with extreme precipitation in previous austral summers over the period (2008-2020) suggests that this combined thermodynamic-dynamic mechanism is present in the majority of such events in Southern Peru and Northern Chile, providing valuable guidance to predict future extreme precipitation events in the Atacama Desert