The Karmanasa River (Hindi: कर्मनाशा नदी) is a tributary of the Ganges. It originates in Kaimur district of Bihar and flows through the Indian states of Uttar Pradesh and Bihar. Along the boundary between Uttar Pradesh and Bihar it has the districts of Sonbhadra, Chandauli, Varanasi and Ghazipur on its left (UP side); and the districts of Kaimur and Buxar on its right (Bihar side).

Karmanasa is also the name of a river in the Garhwal Himalayas.

The name of the river means "destroyer of religious merit". There are several legends about it.

According to one legend, the sage Vishvamitra through tapasya (penance, meditation and correct practices) acquired the power to create a whole new universe. When he set out to create a new universe it aroused consternation in Indra. However, he continued and after creating a copy of our universe, he started creating people, the first being Trishanku whom he decided to send up to rule his new universe. Indra stopped his progress. That is how Trishanku ended up suspended head down in mid-air. The Karmanasa was born out of the saliva dripping from his mouth.