John Cornyn III (born February 2, 1952) is the junior United States Senator from Texas. He is a Republican and was elected to his first term in November 2002, having defeated Democrat Ron Kirk, the former mayor of Dallas, Texas. In the general election of November 4, 2008, he defeated the Democratic State Representative Rick Noriega of Houston for his second term. He was elected Chairman of the National Republican Senatorial Committee for the 111th U.S. Congress.

Cornyn was born in Houston to Atholene Gale Danley and John Cornyn II. He graduated from Trinity University in 1973, where he majored in journalism and was a member of the local fraternity Chi Delta Tau. He earned a J.D. from St. Mary's University School of Law in 1977, and an LL.M. from the University of Virginia School of Law in 1995.

He served in San Antonio for six years as a district judge before being elected as a Republican in 1990 to the Texas Supreme Court, where he served for seven years. From 1999 to 2002, John Cornyn was the Texas Attorney General, the first of thus far only two Republicans to have held the position. In 2005, Cornyn's name was mentioned among possibilities to replace Supreme Court justices Sandra Day O'Connor or William Rehnquist.