Siddharthanagar (Nepali: सिद्धार्थनगर), formerly and colloquially still called Bhairahawa (Nepali: भैरहवा), is a municipality and the administrative headquarter of Rupandehi District in Lumbini Province of Nepal, 265 km (165 mi) west of Nepal's capital Kathmandu. It is the closest city to Lumbini, the birthplace of Gautama Buddha, which is located 25 km (16 mi) to the west. The city borders the Indian city of Sonauli in Maharajganj district of Uttar Pradesh.

Although the current name was first used in 1977, many still refer to it as Bhairahawa.

The city was founded as Bhairahawa in 1967. The city's current name Siddharthanagar derives from Buddha's given name Siddhartha, as the birthplace of Buddha is located only 25 km (16 mi) to the west. The name was changed to Siddhartanagar in 1977 AD by poet Komal Dutta Tiwari.

The highest temperature ever recorded in Siddharthanagar was 45.7 °C (114.3 °F) on 7 June 2023, while the lowest temperature ever recorded was −1.1 °C (30.0 °F) on 20 January 1971.

The population of Siddharthanagar in 2001 was 52,569. In 2011 it was 64,566 and in 2021 it was 74,436.

Bhojpuri is spoken by 42.2%, Nepali by 40.7%, Awadhi by 5.9%, Magar by 1.9% and Hindi by 1.9% of the Siddharthnagar municipality population as their first language.

Rupandehi Lilaram Ma Vi, Bhairahawa Ma Vi, Paklihawa Ma Vi are old higher secondary schools.

There are several higher education facilities in Siddharthanagar including Rupandehi Campus, IAAS Paklihawa Campus, Universal College of Medical Sciences and Bhairahawa Namuna Campus.

• Butwal