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An atmospheric river (AR) impacting Tasmania, Australia, and the Southern Ocean during the austral summer on 28–29 January 2018 during the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study campaign is analyzed using a modeling and observational approach. Gulfstream‐V dropsonde measurements and Global Precipitation Measurement radar analyses were used in conjunction with Weather Research and Forecasting model simulations with water vapor tracers to investigate the relative contributions of tropical and midlatitude moisture sources to the AR. Moisture associated with a monsoonal tropical depression became entrained into a midlatitude frontal system that extended to 60°S, reaching the associated low‐pressure system 850 km off the coast of Antarctica—effectively connecting the tropics and the polar region. Tropical moisture contributed to about 50% of the precipitable water within the AR as the flow moved over the Southern Ocean near Tasmania. The tropical contribution to precipitation decreased with latitude, from >70% over Australia, to ~50% off the Australian coast, to less than 5% poleward of 55°S. The integrated vapor transport (IVT) through the core of the AR reached above 500 kg m−1 s−1 during 1200 UTC 28 January to 0600 UTC 29 January, 1.29 times the average amount of water carried by the world's largest terrestrial river, the Amazon. The high IVT strength might be attributed to the higher water vapor content associated with the warmer temperatures across Australia and the Southern Ocean in austral summer. Plain Language Summary: Atmospheric rivers (ARs) are long, narrow corridors of strong horizontal water vapor transport, normally associated with a low‐level jet stream ahead of a cold front. They play a critical role in the global hydrologic cycle and have been studied extensively over the northeast Pacific. This and a companion paper are the first to analyze an AR event impacting Tasmania, Australia, and the Southern Ocean using a modeling and observational approach. The AR extended from the tropics over northern Australia southeastward to the southern Australian coast and over the Southern Ocean to 60°S. The flow at higher altitudes in the AR was primarily composed of moisture originating in the tropics, while at lower altitudes and warmer temperatures, the moisture was primarily sourced from middle latitudes as a result of the circulation associated with a cold front. The tropical contribution to precipitation decreased with latitude, from >70% over Australia, to ~50% off the Australian coast, to less than 5% poleward of 55°S. Water vapor transported through the core of the AR was more than the average amount of water carried by the world's largest river, the Amazon, during the same time interval. Key Points: The contribution of moisture from the tropics to precipitation within an Australian summer atmospheric river is documentedOver 50% of the moisture arriving at the Southern Ocean at altitudes above the 0°C isotherm was sourced from tropical latitudesThe tropical contribution to precipitation decreased with latitude, >70% over Australia, ~50% off Australia, and <10% poleward of 60°S [ABSTRACT FROM AUTHOR]