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. Under the Illinois Constitution of 1970, the Illinois Senate is made up of 59 senators elected from individual legislative districts determined by population and redistricted every 10 years; based on the 2020 U.S. census each senator represents approximately 213,347 people. Senators are divided into three groups, each group having a two-year term at a different part of the decade between censuses, with the rest of the decade being taken up by two four-year terms. This ensures that the Senate reflects changes made when the General Assembly redistricts itself after each census.

The Illinois General Assembly is the legislature of the U.S. state of Illinois. It has two chambers, the Illinois House of Representatives and the Illinois Senate. The General Assembly was created by the first state constitution adopted in 1818. As of 2023, the current General Assembly is the 103rd; the term of an assembly lasts two years.