John Isner (born April 26, 1985) is an American professional tennis player. At a height of 6-foot-9, he is one of the tallest professional tennis players in the world and is best known for his powerful serves with speeds up to 145 miles per hour. He achieved his career-high rank of no. 11 on February 27, 2012, and was the second-ranked American tennis player behind Andy Roddick or Mardy Fish. His most famous victories were against Roger Federer in the Davis Cup encounter in February 2012, Roddick in the 2009 US Open, and against Nicolas Mahut at the 2010 Wimbledon Championships where he played the longest ever professional tennis match, which lasted 11 hours and 5 minutes over the course of three days.

John Isner beat world No. 1 Novak Djokovic 7-6 (7), 3-6, 7-6 (5) with his powerful serves in a semifinal match at the BNP Parbias Open on March 17, 2012.

A graduate from the University of Georgia, Isner played collegiate tennis from 2004 to 2007 and became the school's all-time leader in singles and doubles victories. He earned All-American honours each of his four years and led University of Georgia to the 2007 NCAA championship.