The Illinois Senate is the upper chamber of the Illinois General Assembly, the legislative branch of the government of the state of Illinois in the United States. The body was created by the first state constitution adopted in 1818. The Illinois Senate is made up of 59 senators elected from individual legislative districts determined by population. In order to avoid complete turnovers in Senate membership, under the Illinois Constitution of 1970, some senators are elected to two-year terms while others are elected to four-year terms. Senate districts are divided into three groups. One or two of these groups are elected every two years for either a two or four year term. In contrast, the Illinois House of Representatives is made of 118 members with its entire membership elected to two-year terms.

The Illinois Senate convenes at the Illinois State Capitol in Springfield, Illinois. Its first official working day is the second Wednesday of January each year. Its primary duties are to pass bills into law, approve the state budget, confirm appointments to state departments and agencies, act on federal constitutional amendments and propose constitutional amendments for Illinois. It also has the power to override gubernatorial vetoes through a three-fifths majority vote. The Illinois Senate tries impeachments made by the House of Representatives, and can convict impeached officers by a two-thirds vote.

The Democratic Party of Illinois currently holds a veto-proof majority in the Illinois Senate.