Climate change is associated with changes in the frequency and intensity of extreme weather events. Extreme weather is impacting the mass balance of Andean glaciers, a phenomenon that requires further detailed investigation. Among these extreme events, atmospheric rivers (ARs) play a significant role, potentially leading to either accumulation or melting events on glaciers. To assess the impact of ARs on Andean glaciers, we analysed an unseasonal event that occurred at the end of January 2021 - marked by extreme snowfall in the highlands and heavy rainfall, landslides and flash floods in the lowlands - during the typically dry austral summer period. Satellite imagery and meteorological observations in the glaciated Maipo River basin and its Olivares River sub-basin (33 degrees S) enabled the characterisation of this event and its basin-scale impacts. Moreover, a glacier mass balance model allows us to quantify the effects of the AR on the Olivares Alfa Glacier (4284 to 4988 m a.s.l.) in the context of the preceding 6 hydrological years. The large water vapour transport by the AR led to substantial snow accumulation on the Maipo River glaciers, resulting in a post-event snow line observed at 2463 m a.s.l. In the Olivares River sub-basin, the 0 degrees C isotherm dropped from typical summertime altitudes of 4000-4500 m a.s.l. to 3250 m a.s.l. during the event, below the frontal zone of all glaciers in this sub-basin. The mass balance model for the Olivares Alfa Glacier during the dry 2020-2021 hydrological year showed a trend toward negative values at the beginning of the ablation season, aligned with previous years and the prevailing severe drought conditions. However, the AR snowfall event, combined with cooler conditions and other small accumulation events during the remainder of the ablation season compared to previous years, offset this trend and brought the mass balance closer to equilibrium. This demonstrates that an unseasonal snow accumulation event can significantly counteract the broader seasonal trends affecting subtropical Andean glaciers. Our study sheds light on the impacts of extreme and unseasonal snow accumulation events on glacier mass balance in the high Andes, particularly those associated with ARs, a synoptic feature projected to become more common in a warming climate.