The Yamuna (pronounced [jəmʊnɑː]; IAST: Yamunā) is the second-largest tributary river of the Ganges by discharge and the longest tributary in India. Originating from the Yamunotri Glacier at a height of about 4,500 m (14,800 ft) on the southwestern slopes of Bandarpunch peaks of the Lower Himalaya in Uttarakhand, it travels 1,376 kilometres (855 mi) and has a drainage system of 366,223 square kilometres (141,399 sq mi), 40.2% of the entire Ganges Basin. It merges with the Ganges at Triveni Sangam, Prayagraj, which is a site of the Kumbh Mela, a Hindu festival held every 12 years.

Like the Ganges, the Yamuna is highly venerated in Hinduism and worshipped as the goddess Yamuna. In Hinduism, she is believed to be the daughter of the sun god, Surya, and the sister of Yama, the god of death, and so she is also known as Yami. According to popular Hindu legends, bathing in Yamuna's sacred waters frees one from the torments of death.

The river crosses several states such as Haryana, Uttar Pradesh, Uttarakhand and Delhi. It also meets several tributaries along the way, including Tons, Chambal, its longest tributary which has its own large basin, followed by Sindh, the Betwa, and Ken. From Uttarakhand, the river flows into the state of Himachal Pradesh. After passing Paonta Sahib, Yamuna flows along the boundary of Haryana and Uttar Pradesh and after exiting Haryana it continues to flow till it merges with the river Ganges at Sangam or Prayag in Prayagraj (Uttar Pradesh). It helps create the highly fertile alluvial Ganges-Yamuna Doab region between itself and the Ganges in the Indo-Gangetic plain.

Nearly 57 million people depend on the Yamuna's waters, and the river accounts for more than 70 percent of Delhi's water supply. It has an annual flow of 97 billion cubic metres, and nearly 4 billion cubic metres are consumed every year (of which irrigation constitutes 96%). At the Hathni Kund Barrage, its waters are diverted into two large canals: the Western Yamuna Canal flowing towards Haryana, and the Eastern Yamuna Canal flowing towards Uttar Pradesh. Beyond that point the Yamuna is joined by the Somb, a seasonal rivulet from Haryana, and by the highly polluted Hindon River near Noida, by Najafgarh drain near Wazirabad and by various other drains, so that it continues only as a trickling sewage-bearing drain before joining the Chambal at Pachnada in the Etawah District of Uttar Pradesh.

The water quality in Upper Yamuna, as the 375-kilometre (233 mi) long stretch of Yamuna is called from its origin at Yamunotri to Okhla barrage, is of "reasonably good quality" until the Wazirabad barrage in Delhi. Below this, the discharge of wastewater in Delhi through 15 drains between Wazirabad barrage and Okhla barrage renders the river severely polluted. Wazirabad barrage to Okhla Barrage, 22 km (14 mi) stretch of Yamuna in Delhi, is less than 2% of Yamuna's total length but accounts for nearly 80% of the total pollution in the river. Untreated wastewater and poor quality of water discharged from the wastewater treatment plants are the major reasons of Yamuna's pollution in Delhi. To address river pollution, measures have been taken by the Ministry of Environment and Forests (MoEF) under the Yamuna Action Plan (YAP) which has been implemented since 1993 by the MoEF's National River Conservation Directorate (NRCD).

The present Sarsuti river which originates in the Shivalik hills in Himachal and Haryana border and merges with Ghaggar River near Pehowa is the palaeochannel of Yamuna. Yamuna changed its course to the east due to a shift in the slope of the Earth's crust caused by plate tectonics.

The source of Yamuna lies in the Yamunotri Glacier at an elevation of 6,387 metres (20,955 ft), on the southwestern slopes of Banderpooch peaks, which lie in the Mussoorie range of the Lower Himalayas, north of Haridwar in Uttarkashi district, Uttarakhand. Yamunotri temple, a shrine dedicated to the goddess Yamuna, is one of the holiest shrines in Hinduism, and part of the Chota Char Dham Yatra circuit. Also standing close to the temple, on its 13-kilometre (8 mi) trek route that follows the right bank of the river, lies Markendeya Tirtha, where the sage Markandeya wrote the Markandeya Purana.

The river flows southwards for about 200 kilometres (120 mi), through the Lower Himalayas and the Shivalik Hills Range. Morainic deposits are found along the steep Upper Yamuna, highlighted with geomorphic features such as interlocking spurs, steep rock benches, gorges and stream terraces. Large terraces formed over a long period of time can be seen in the lower course of the river, such as those near Naugoan. An important part of its early catchment area, totalling 2,320 square kilometres (900 sq mi), lies in Himachal Pradesh. The Tons, Yamuna's largest tributary, drains a large portion of the upper catchment area and holds more water than the main stream. It rises from the Hari-ki-dun valley and merges after Kalsi near Dehradun. The drainage system of the river stretches between Giri-Sutlej catchment in Himachal and Yamuna-Bhilangna catchment in Garhwal, also draining the ridge of Shimla. Kalanag (6,387 metres [20,955 ft]) is the highest point of the Yamuna basin. Other tributaries in the region are the Giri, Rishi Ganga Kunta, Hanuman Ganga and Bata, which drain the upper catchment area of the Yamuna basin.