Siddharthnagar, also known as Naugarh, is a city and the district headquarters of Siddharthnagar district in the Purvanchal region of Uttar Pradesh, India.

The city is named after Prince Siddhartha, the childhood name of Gautama Buddha. The nearby archaeological site of Kapilavastu is associated with the early life of the Buddha.

Siddharthnagar district was established on 29 December 1988 after being separated from Basti district. Naugarh became the district headquarters following the district's formation.

The region historically formed part of the ancient Shakya republic and is closely linked with Buddhist heritage sites around Kapilavastu and Piprahwa.

According to the 2011 Census of India, Siddharthnagar city had a population of 25,422, including 13,071 males and 12,351 females.

Hindi is the official language of Siddharthnagar, while Urdu is also widely used in administration, education, literature, and local communication.

Regional languages spoken in the area include Bhojpuri and Awadhi.

Urdu has a notable literary and cultural presence in Siddharthnagar district and is commonly spoken among local communities.