In American and Canadian football, a guard (G) is a player that lines up between the center and the tackles on the offensive line of a football team.

The guard's job is to protect the quarterback from the oncoming defensive line and linebackers during pass plays, as well as creating openings (holes) for the running backs to head through. Guards perform speed blocking and "pulling"—sprinting out in front of a running back in order to block for him. Guards are automatically considered ineligible receivers, so they cannot touch a pass, unless it is to recover a fumble or is first touched by a defender or eligible receiver.

Guards, like other linemen, today are often over 300 pounds / 135 kg. Currently, the heaviest starting guard is Toniu Fonoti of the Atlanta Falcons, weighing 390 pounds (177 kg). The lightest is Pete Kendall at 280 pounds (127 kg) (formerly of the Seattle Seahawks, Arizona Cardinals and New York Jets, now playing for the Washington Redskins). To date, no lineman over 300 pounds has ever been elected to the Pro Football Hall of Fame, but linemen of this mass have become prevalent only since the mid-1980s.

In his book with Dick Schaap titled Instant Replay, Green Bay Packers' right guard Jerry Kramer wrote about the trials and tribulations of an NFL guard in 1967, the final year he was coached by Vince Lombardi. Lombardi himself was a right guard (undersized, even in his era, at 5'8", 185 lb. / 173 cm, 84 kg) for Fordham's famed Seven Blocks of Granite in the mid 1930s. Kramer was perhaps the finest pulling guard of all time. Teamed with left guard Fuzzy Thurston, this tandem was the cornerstone of the famed Packer Sweep of the 1960s. Kramer also received notoriety because of his key block in the final seconds (with center Ken Bowman) on Dallas' Jethro Pugh in the Ice Bowl, securing the Packers' still unmatched third consecutive NFL championship.