The district was named after prince Siddhartha, the pre-enlightenment name of Buddha, as he spent his early years (till the age of 29 years) in Kapilavastu, parts of which included territory in this district.Siddharthnagar District lies between 27°N to 27°28'N and 82°45'E to 83°10'E. The district borders Nepal's district Kapilvastu on the north and Rupandehi on the northeast. Otherwise it is surrounded by other districts of Uttar Pradesh: Maharajganj on the east, Basti and Sant Kabir Nagar on the south, and Balrampur on the west. Siddarthnagar's area is 2,895 km sqaure.

Siddharthnagar district comprises 5 tehsils;
• Naugarh (Sadar Tehsil)
• Shohratgarh,
• Bansi,
• Itwa and
• Domariyaganj.
5 Vidhan Sabha constituencies are located within this district. These are Shohratgarh, Kapilvastu, Bansi, Itwa and Domariyaganj. The only Lok Sabha constituency in this district is the Domariyaganj.

According to the 2011 census Siddharthnagar district has a population of 25,59,297. The district has a population density of 884 inhabitants per square kilometre. Its population growth rate over the decade 2001-2011 was 25.4%.Siddharth Nagar has a sex ratio of 976 females for every 1000 males, and a literacy rate of 59.2.