The River Torrens /ˈtɒrənz/ (Karrawirra Parri / Karrawirraparri) is the most significant river of the Adelaide Plains. It was one of the main reasons for the siting of the city of Adelaide, capital of South Australia. It flows 85 kilometres (53 mi) from its source in the Adelaide Hills near Mount Pleasant, across the Adelaide Plains, past the city centre and empties into Gulf St Vincent between Henley Beach South and West Beach. The upper stretches of the river and the reservoirs in its watershed supply a significant part of the city's water supply.

The river is also known by the native Kaurna name for the river—Karrawirra Parri or Karrawirraparri (karra meaning redgum, wirra meaning forest and parri meaning river), having been officially dual-named in 2001. Another Kaurna name for the river was Tarndaparri (Kangaroo river). The river was thought to be a reflection of the Milky Way ("wodliparri"), and was the heartland of the Kaurna people, who lived along its length and around the tributary creeks.

At its 1836 exploration by William Light, an inland bend was chosen as the site of the Adelaide city centre and North Adelaide. The river was first named the Yatala by the initial exploration party, but later renamed to honour Robert Torrens senior, chairman of the board of Colonisation Commissioners for South Australia from 1834 to 1841 (when he was sacked). From March 1837 settlers camped in tents and makeshift huts along the west end of the River Torrens and freely used the river's resources. A Native Location was created on the north banks of the Torrens and indigenous labour was often used by the settlers for tasks such as hewing wood or delivering water. During the early years of settlement, the river acted as both the city's primary water source and main sewer, leading to outbreaks of typhoid.

Since European settlement the river has been a frequently touted tourist attraction. The river's long linear parks and a constructed lake in the lower stretch are an icon of the city. The river's flora and fauna have been both deliberately and accidentally impacted since settlement. In the 19th century, native forests were cleared, gravel removed for construction and many foreign species introduced. With construction of the linear parks, many species native to the river have been replanted, and introduced species have been controlled as weeds.

The river and its tributaries are highly variable in flow, and together drain an area of 508 square kilometres (196 sq mi). They range from sometimes raging torrents, damaging bridges and flooding city areas, to trickles and completely dry in summer. Winter and spring flooding has prompted the construction of flood reduction works. A constructed sea outlet, landscaped linear parks and three holding reservoirs contain peak flow.

The River Torrens runs largely westward from the Adelaide Hills, through the centre of Adelaide to the Gulf St Vincent. It originates close to the eastern fault scarp of the Mount Lofty Ranges, near Mount Pleasant, approximately 480 metres (1,575 ft) above sea level. It runs predominantly along faulted north-south ground structures, which were formed over 250 million years ago during the Paleozoic era then further dislocated during the Cretaceous and earliest Tertiary. There is a 400-metre (1,300 ft) subsidence along the Para Fault which also affects the rivers flow. This subsidence was formed in the last two million years, after the Pliocene era.

From its origin to Birdwood the river follows rolling, relatively level country before entering a hilly section that continues to Gumeracha. The river then follows sedimentary rock strata before entering a gorge after Cudlee Creek. It flows through the gorge to Athelstone, passing over the Eden Fault Zone of the Adelaide Hills face and associated escarpment. After the scarp it flows over sedimentary rocks of varying resistance to erosion, which has led to interspersed narrows and broad basins. From the base of the Adelaide Hills to the Adelaide central business district it runs in a shallow valley with a terraced floor, then down the slope of its own alluvial fan. The structure of this fan shows that the river formerly entered Gulf St Vincent via the Port River. Over time the Torrens deposited sediment, choking its own outflow; becoming locked behind coastal sand dunes and forming the swampy areas of the Cowandilla Plains and The Reedbeds.

The Torrens is fed by numerous seasonal creeks, which are dry for most of the year in their lower reaches, but prone to occasional flooding during the winter and spring. There are five main creeks that join it from the southern side as it crosses the Adelaide Plains east of Adelaide, and at least five more in its path through the Adelaide Hills.