The Sarayu (also Sarju; Dev. सरयु saráyu- f., later Dev. सरयू sarayū-) is a river that flows through the Indian state of Uttar Pradesh. This river is of ancient significance, finding mentions in the Vedas and the Ramayana. The Sarayu forms at the confluence of the Karnali (or Ghaghara) and Mahakali (or Sharda) in Bahraich District. The Mahakali or Sharda forms the Indian-Nepalese border.Ayodhya is situated on the banks of river Sarayu.
On Ram Navami, the festival that celebrates the birthday of Lord Rama, thousands of people take a dip in the Sarayu River at Ayodhya.
The name is the feminine derivative of the Sanskrit root सर् sar "to flow"; as a masculine stem, saráyu- means "air, wind", i.e. "that which is streaming".
The river is mentioned three times in the Rigveda. The banks of the Sarayu are the location of the slaying of two Aryas at the hands of Indra in RV 4.30.18. It is listed together with western tributaries to the Indus: Rasā, Anitabha, Kubha, Krumu, and the Sindhu itself as obstacles crossed by the Maruts in RV 5.53.9. In this verse, Purisini appears as its epithet. At this stage of the earlier Rigveda, it apparently was a river west of the Indus system that corresponds to Iranian Harayu (Avestan acc. Harōiium, Old Persian Haraiva, modern Harē), the Herat river. It is invoked together with Sindhu and Sarasvati (two of the most prominent Rigvedic rivers) in the late hymn RV 10.64.