Duane Charles "Bill" Parcells (born August 22, 1941) is an American former football coach who served as a head coach in the National Football League (NFL) for 19 seasons. He rose to prominence as the head coach of the New York Giants from 1983 to 1990, where he won two Super Bowl titles. Parcells was later the head coach of the New England Patriots from 1993 to 1996, the New York Jets from 1997 to 1999, and the Dallas Cowboys from 2003 to 2006. Nicknamed "the Big Tuna", he is the only NFL coach to lead four different franchises to the playoffs and three to a conference championship game.

As the head coach of the Giants, Parcells took over a franchise that had qualified for the playoffs only once in the past decade and had only one winning record in their last 10 seasons. Within four years, he guided them to their first Super Bowl title and won a second championship in Super Bowl XXV four years later. Parcells retired following the second Super Bowl, but came out of retirement in 1993 to become the head coach of the Patriots, another struggling franchise at the time. By his fourth season, New England reached Super Bowl XXXI, although the game ended in defeat. Parcells left the Patriots after their Super Bowl loss and became the head coach of the Jets, who went from a one-win season to appearing in the AFC Championship Game by his second year. He retired for a second time in 1999, but returned again in 2003 as the head coach of the Cowboys. The Cowboys made two playoff appearances under Parcells, although both ended in first-round defeats, leading to his third and final retirement in 2007.

Following his final retirement from coaching, Parcells has served as the vice president of football operations with the Miami Dolphins, a position he held from 2008 to 2010. Parcells was inducted to the Pro Football Hall of Fame in 2013.