thumb|right|Sutlej River is tributary to Indus River
The Sutlej River (alternatively spelled as Satluj River) (, , , , and ) is the longest of the five rivers that flow through the historic crossroad region of Punjab in northern India and Pakistan. It is located north of the Vindhya Range, south of the Hindu Kush segment of the Himalayas, and east of the Central Sulaiman Range in Pakistan.

The Sutlej is sometimes known as the Red River. It is the easternmost tributary of the Indus River. Its source is at Lake Rakshastal in Tibet near Mount Kailas, and it flows generally west and southwest entering India through the Shipki La pass in Himachal Pradesh. It waters the ancient and historically important region of Greater Punjab. The region to its south and east is arid, and is known as the Great Indian Desert or Thar Desert.

The Sutlej joins with the Beas River in Hari-Ke-Patan, Amritsar, Punjāb, India, and continues southwest into Pakistan to unite with the Chenab River, forming the Panjnad River south of ancient Multān. The Panjnad joins the Indus River at Mithankot. Indus then flows through a gorge near Sukkur, flows through the fertile plains region of Sindh, and terminates in the Arabian Sea near the port city of Karachi in Pakistan.

The waters of the Sutlej are allocated to India under the Indus Waters Treaty between India and Pakistan, and are mostly diverted to irrigation canals in India. A huge, multipurpose Bhakra-Nangal Dam has been built on the Sutlej by the Indian government. There are several major hydroelectric projects on the Sutlej, e.g. the 1000MW Karcham-Wangtoo HEP. There has been a proposal to build a long heavy freight canal, known as the (SYL), in India to connect the Sutlej and Yamuna rivers. However, the proposal met obstacles and was referred to the Supreme Court.

The Sutlej was known as Śutudri in the Vedic period.

Geology of the Sutlej

The Sutlej, along with all of the Punjab rivers, is thought to have drained east into the Ganges prior to 5 mya.

There is substantial geologic evidence to indicate that prior to 1700 BC at the latest, Sutlej was an important tributary of the Ghaggar-Hakra River (possibly through the Saraswati river) rather than the Indus with various authors putting the redirection from 2500-2000 BC or 5000-3000 BC. Geologists believe that tectonic activity created elevation changes which redirected the flow of Sutlej from the southeast to the southwest. The mighty Saraswati then began to dry up, causing desertification of Cholistan and the eastern part of the modern state of Sindh. The desertification resulted in abandonment of numerous ancient human settlements along the banks of Saraswati.

There is some evidence that the high rate of erosion caused by the modern Sutlej River has influenced the local faulting and rapidly exhumed rocks above Rampur. This would be similar to, but on a much smaller scale then, the exhumation of rocks by the Indus River in Nanga Parbat, Pakistan. The Sutlej river also exposes a doubled inverted metamorphic gradient.

The source of the Sutlej is just west of Mt. Kailash in western Tibet. This is roadless area, and was first explored by kayak and raft by Russian and German teams in 2004.

The largest modern industrial city along the Sutlej banks is Ludhiana.