The district is located in the eastern part of the Outer Terai and covers an area of 1,257 km2 (485 sq mi).

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.

[3] These are as follows:[4] According to the 2021 Nepal census, 926962 people reside in Sunsari in 212545 households with average family size of 4.36.

[7] Ethnicity/caste: 12.1% were Tharu, 11.5% Musalman, 9.2% Chhetri, 7.9% Hill Brahmin, 6.6% Rai, 4.3% Yadav, 4.0% Koiri/Kushwaha, 3.8% Newar, 3.4% Musahar, 3.2% Limbu, 3.1% Jhangad/Dhagar, 2.3% Tamang, 2.3% Teli, 2.2% Kami, 2.0% Magar, 1.6% Dhanuk, 1.5% Bantar/Sardar, 1.2% Chamar/Harijan/Ram, 1.1% Halwai, 1.0% Damai/Dholi, 1.0% Gurung, 0.8% Kathabaniyan, 0.8% Khatwe, 0.6% Gharti/Bhujel, 0.6% Majhi, 0.6% Mallaha, 0.6% Marwadi, 0.6% other Terai, 0.5% Terai Brahmin, 0.5% Dhimal, 0.5% Hajam/Thakur, 0.5% Sanyasi/Dasnami, 0.4% Dusadh/Paswan/Pasi, 0.4% Kulung, 0.4% Sarki, 0.3% Bantawa, 0.3% Bengali, 0.3% Gaderi/Bhedihar, 0.3% Kalwar, 0.3% Kewat, 0.3% Khawas, 0.3% Sarbaria, 0.3% Tatma/Tatwa, 0.2% Badhaee, 0.2% Chamling, 0.2% Dom, 0.2% Kayastha, 0.2% Kumal, 0.2% Kumhar, 0.2% Rajbanshi, 0.2% Rajput, 0.2% Sherpa, 0.2% Sudhi, 0.2% Sunuwar, 0.1% Amat, 0.1% Badi, 0.1% Baraee, 0.1% Bhote, 0.1% Danuwar, 0.1% Dhobi, 0.1% Kurmi, 0.1% Lohar, 0.1% Munda, 0.1% Nuniya, 0.1% Pattharkatta/Kushwadiya, 0.1% Punjabi/Sikh, 0.1% Rajbhar, 0.1% Rajdhov, 0.1% Sonar, 0.1% Thakuri, 0.1% Thulung, 0.1% Yakkha and 0.3% others.