Mairwa is a city located in Siwan District of the eastern state of India, Bihar. Also known as Mairwa Dham, it is located in the west of Siwan, the administrative headquarters of the district at a distance of 17 km. The town is located between 26.23° North latitude and 84.15° East longitude at a height of 65 metres (213 feet) above mean sea level. The major access points are the airport at Patna Mairwa Railway Station on Chhapra-Gorakhpur rail route.

According to the Indian Census report of 2001, Mairwa had a population of 18,696. Males account for 53% of the population while females constitute 47% of the population. In Mairwa, 17% of the population is less than six years of age. The average literacy rate of Mairwa is 54% which is much lower than the national average literacy rate which is 59.5%. Out of it, the male literacy rate is 64% and the female literacy rate is 42%.

The major attraction of the town is Brahma Asthan shrine, which is built over the remnants of Saint Hari Baba. The shrine is also known as Hari Baba ka Asthan. There is a beautiful shrine on the bank of the Jharhi River where a fair is held annually during Kartik and Chaitra months.