Kenneth Robert Buck (born February 16, 1959) is an American lawyer and politician who represented Colorado's 4th congressional district in the United States House of Representatives from 2015 until his resignation in 2024. Buck served as chair of the Colorado Republican Party, from 2019 to 2021. Formerly the District Attorney for Weld County, Colorado, Buck ran unsuccessfully for U.S. Senate in 2010, losing to Democrat Michael Bennet.

In Congress, Buck joined the Freedom Caucus, and emerged as a staunch fiscal conservative, as well as one of the foremost proponents of antitrust enforcement in the Republican Party.

Buck announced in November 2023 that he would not seek a sixth House term, stating that his party's "insidious narratives breed widespread cynicism and erode Americans' confidence in the rule of law." On March 12, 2024, Buck announced he would resign from Congress at the end of the following week on March 22, 2024. Governor Jared Polis scheduled the special election for Buck's replacement for June 25.

Buck was born in Ossining, New York, in 1959. He and his two brothers were encouraged by their parents, Ruth (Larsen) and James Buck, both New York lawyers, to attend Ivy League colleges. Buck earned a Bachelor of Arts degree in politics from Princeton University in 1981 with a 75-page long senior thesis titled "Saudi Arabia: Caught Between a Rock and a Hard Place". Buck later said that the Princeton degree was "more important to [my father] than me".

At Princeton, Buck played four years of football on the Princeton Tigers football team, including one year as a defensive back/punter/kicker and three years as a punter, earning All-Ivy League honors as a punter his senior year. After college, he worked in Wyoming at the state legislative services office and received a Juris Doctor from the University of Wyoming College of Law in 1985. He was also an instructor at the University of Denver Law School and for the National Institute for Trial Advocacy in Colorado.

In 1986, Buck was hired by Congressman Dick Cheney to work on the Iran–Contra investigation. Following that assignment, he worked as a prosecutor with the U.S. Department of Justice in Washington D.C.

In 1990, Buck joined the United States Attorney's Office for the District of Colorado, where he became Chief of the Criminal Division. Buck was formally reprimanded and required to take ethics classes in 2001 for a meeting he had with defense attorneys about a felony case he thought should not be pursued. Only one of the three men initially indicted on felony charges was convicted for a misdemeanor offense. Buck said he is "not proud" of the incident that effectively ended his career with the Justice Department, but that he felt it was unethical to prosecute such a "weak" case. One of the three men donated $700 to Buck's 2010 Senate campaign.

Buck was elected the District Attorney for Weld County, Colorado, in 2004. When he suspected that Social Security numbers were being stolen by undocumented immigrants, he raided a tax service in Greeley, Colorado, and seized more than 5,000 tax files. The American Civil Liberties Union sued Buck's office for violating the privacy of the service's clients and after an appeal to the Colorado Supreme Court, costing the county approximately $150,000, the raid was deemed unconstitutional. Buck has said that his time enforcing laws for the Justice Department and Weld County stoked his desire to become a lawmaker himself.