James Robert Thompson Jr. (May 8, 1936 – August 14, 2020) was an American politician and federal prosecutor who served as the 37th governor of Illinois from 1977 to 1991. He was Illinois's longest-serving governor, having been elected to four consecutive terms and holding the office for 14 years, and is also the only governor to have served more than two terms.

Thompson was known as a "Rockefeller Republican", governing during his tenure as both a fiscal conservative and social liberal. In later years, Thompson served as a member of the 9/11 Commission, charged with investigating the September 11 attacks.

Thompson was born in Chicago, the son of Agnes Josephine (Swanson) and James Robert Thompson, a physician. His maternal grandparents were Swedish, and his father was descended from colonial Massachusetts governor David Thompson through an entirely paternal line. Through his father's father he is also descended from Josiah Winslow, John Winthrop the Younger, John Leverett, John Underhill and John Mason. His father's mother had ancestors who were Presbyterians of Scottish descent from Carrickfergus, Ballymena and Coleraine, County Antrim, Ireland in what has since become Northern Ireland. Thompson graduated from North Park Academy (now North Park University) and studied at the University of Illinois at Chicago Navy Pier campus before transferring to Washington University in St. Louis, where he earned his undergraduate degree. He received his J.D. from Northwestern University in 1959.

Thompson worked in the Cook County State's Attorney's office, taught at Northwestern University's law school, and was appointed by President Richard Nixon to serve as U.S. Attorney for the Northern District of Illinois. He served from November 1971 until June 1975.

As a federal prosecutor in the early 1970s, he obtained a conviction against former governor Otto Kerner Jr. for his use of improper influence on behalf of the racetrack industry.

He also tried and convicted many of Chicago mayor Richard J. Daley's top aides, most notably Alderman Thomas E. Keane and County Clerk Matt Danaher, on various corruption charges. People like Keane and Danaher, the mayor's point man on patronage were also major figures in the Cook County Democratic Party's political machine. These high-profile cases gave Thompson the celebrity that fueled his run for governor in 1976.

To the chagrin of many, Thompson was bipartisan in his attacks on corruption in Cook County and Chicago. He not only prosecuted high-profile Democrats, but also prominent Republicans such as County Commissioner Floyd Fulle and former U.S. Senate candidate, William Rentschler. Organized crime in Chicago was harder for his unit to crack and there were few high-profile cases during his era.

In the 1976 election, he won 65 percent of the vote over Democratic Secretary of State Michael Howlett, who had defeated incumbent governor Dan Walker in the primary and who had the support of Chicago Mayor and Cook County Democratic Party chairman Richard J. Daley. Thompson was the first candidate for governor to receive over 3 million votes; his tally of 3,000,395 remains the largest number of votes ever cast for a candidate in an election for Governor of Illinois. His first term was for only two years because Illinois moved its gubernatorial election from presidential-election years to midterm-election years.