Siwan is one of the 243 assembly constituencies in Bihar, located in the district bearing the same name. It also forms part of one of the 40 Lok Sabha constituencies from the state.

The current MLA from Siwan is Awadh Bihari Chaudhary of the Rashtriya Janata Dal (RJD), who won the 2020 assembly election with 76,785 votes, defeating Om Prakash Yadav of the Bharatiya Janata Party (BJP) by a margin of 1,973 votes.

According to Election Commission data for 2024, the Sahebganj constituency has a total of 320,094 registered electors, including 167,270 male, 152,819 female and 5 third-gender voters. In comparison, the 2020 electoral roll listed 311,523 electors—163,489 male, 148,028 female and 6 third-gender voters. In 2015, there were 280,805 registered electors, comprising 151,835 males, 128,967 females and 3 from the third gender.

The voter turnout in the 2020 election was recorded at 54.43 per cent, with 169,568 of the 311,523 electors casting their votes. In 2015, turnout stood slightly higher at 56.17 per cent, as 157,722 of the 280,805 registered voters exercised their franchise.

Polling for this constituency in the upcoming Bihar Assembly Elections will take place in the first phase on November 6, 2025.

Siwan constituency spans an area of approximately 165.24 square kilometres, with geographic coordinates of 26°16'44.4"N and 84°20'46.7"E. It shares borders with five other assembly constituencies—Gopalganj, Barharia, Raghunathpur, Ziradei and Hathua.