Sunsari District (Nepali: सुनसरी जिल्ला) is one of 14 districts in Province No. 1 of Nepal. The district is located in the eastern part of the Outer Terai and covers an area of 1,257 km2 (485 sq mi). According to the 2011 Nepal census, the population was 753,328. The district headquarters is located in Inaruwa.

The area was originally part of Morang District but became its own district in 1962 when Nepal was divided into 14 zones and 75 districts. Major cities in Sunsari district are Inaruwa], Itahari, Jhumka, Dharan, and Duhabi. Some religious places of this district are Budha Subba Temple, Ramdhuni, Chataradham, Baraha, Bishnupaduka, Dantakali, and Pindeshor Babadham.

The lowlands of Limbuwan, present day Sunsari, Morang and Jhapa Districts was collectively known as Morang District since the time of King Mung Mawrong Hang of the 7th century.