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 of 59 senators elected from individual legislative districts. In order to avoid complete turnovers in senate membership, some senators are elected to two-year terms while others are elected to four-year terms. 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 Democratic Party of Illinois currently holds a majority in the Illinois Senate.