Nag Nathaiya is a traditional theatrical performance that takes place in the city of Varanasi, located in the northern Indian state of Uttar Pradesh. The performance is a reenactment of the childhood pastimes of Lord Krishna, a popular Hindu deity.

The story of Nag Nathaiya revolves around an incident where young Krishna, while playing with his friends, accidentally releases the river serpent, Kaliya, from the Yamuna River. Kaliya had been poisoning the river and causing harm to the surrounding community. Krishna confronts Kaliya and defeats him, thus freeing the river from his poison and restoring peace to the region.

The Nag Nathaiya performance is typically staged during the Hindu festival of Diwali, which usually falls in October or November. The performance involves elaborate sets, costumes, and music, and is attended by thousands of people each year. It is considered a significant cultural event in Varanasi and is an important part of the city's religious and artistic heritage.