Mark David McGwire (born October 1, 1963), nicknamed "Big Mac", is an American former professional baseball first baseman who played 16 seasons in Major League Baseball (MLB) from 1986 to 2001 for the Oakland Athletics and the St. Louis Cardinals. He won two World Series championships, one with Oakland as a player in 1989 and one with St. Louis as a coach in 2011. One of the most prolific home run hitters in baseball history, McGwire hit 583 home runs during his career, which ranked 5th-most in MLB history at the time of his retirement and currently ranks 11th. He holds the major-league career record for at bats per home run ratio (10.6), and is the former record holder for both home runs in a single season (70 in 1998) and home runs hit by a rookie (49 in 1987).

McGwire led the major leagues in home runs in five different seasons, and set the major-league record for home runs hit in a four-season period from 1996 to 1999 with 245. He demonstrated exemplary patience as a batter, producing a career .394 on-base percentage (OBP) and twice leading the major leagues in bases on balls. McGwire also led the league in runs batted in once, on-base percentage twice, and slugging percentage four times. Injuries cut short even greater potential, as he reached 140 games played in just eight of 16 total seasons. Injuries particularly cut into his playing time in 2000 and 2001 and factored into his decision to retire. A right-handed batter and thrower, McGwire stood 6 feet 5 inches (1.96 m) tall and weighed 245 pounds (111 kg) during his playing career.

With the Cardinals in 1998, McGwire joined Cubs slugger Sammy Sosa in a chase for the single-season home-run record set by Roger Maris in 1961. McGwire surpassed Maris and finished with 70 home runs, a record that Barry Bonds would break three years later with 73.

McGwire was one of several central figures in baseball's steroids scandal. In 2010, McGwire publicly admitted to using performance-enhancing drugs during a large portion of his career.