John Dennis Hastert (; born January 2, 1942) is an American politician, former educator, and convicted felon who represented Illinois's 14th congressional district from 1987 to 2007 and served as the 51st Speaker of the United States House of Representatives from 1999 to 2007. Hastert was the longest-serving Republican Speaker of the House in history. After Democrats gained a majority in the House in 2007, Hastert resigned and began work as a lobbyist. In 2016, he was sentenced to 15 months in prison for financial offenses related to the sexual abuse of teenage boys.

From 1965 to 1981, Hastert was a high school teacher and coach at Yorkville High School in Yorkville, Illinois. He lost a 1980 bid for the Illinois House of Representatives but ran again and won a seat in 1981. He was first elected to the United States House of Representatives in 1986 and was re-elected every two years until he retired in 2007. Hastert rose through the Republican ranks in the House, becoming chief deputy whip in 1995 and speaker in 1999. As Speaker of the House, Hastert supported the George W. Bush administration's foreign and domestic policies. After Democrats took control of the House in 2007 following the 2006 elections, Hastert declined to seek the position of minority leader, resigned his House seat, and became a lobbyist at the firm of Dickstein Shapiro.

In May 2015, Hastert was indicted on federal charges of structuring bank withdrawals to evade bank reporting requirements and making false statements to federal investigators. Federal prosecutors said that the funds withdrawn by Hastert were used as hush money to conceal his past sexual misconduct. In October 2015, Hastert entered into a plea agreement with prosecutors. Under the agreement, Hastert pleaded guilty to the structuring charge (a felony); the charge of making false statements was dropped. In court submissions filed in April 2016, federal prosecutors alleged that Hastert had molested at least four boys as young as 14 years of age during his time as a high school wrestling coach. At a sentencing hearing, Hastert admitted that he had sexually abused boys whom he had coached. Referring to Hastert as a "serial child molester", a federal judge imposed a sentence of 15 months in prison, two years' supervised release, and a $250,000 fine. Hastert was imprisoned in 2016 and was released 13 months later. He became the highest-ranking elected official in U.S. history to serve a prison sentence.