In American football, "the twelfth man" is a phrase often used to refer to the home spectators. The home team coach might acknowledge a particularly loud and boisterous crowd in a post-game interview by saying "That crowd was really a twelfth man for us out there tonight."

American football is perhaps uniquely suited to crowd intervention, as a play is started by a series of verbal signals by the quarterback. At the very least, these signals indicate when the play should start (on the nth repetition of the syllable "hut"), and thus a mis-hearing could cause a false start along the line. At worst, in an audible situation the QB is actually changing the play at the line, and thus the offensive players need to be able to understand what is being yelled. A good crowd will make noise while their team is on defense, trying to disrupt the visitors' signals; this is particularly common on third down situations when the offense needs to advance a certain distance to continue its drive.

The most well-known "twelfth man" tradition in football is that of Texas A&M University. This tradition dates back to a 1922 game vs. Centre College. In those days, players played both offense and defense, and substitutions were fairly limited, so coaches did not see much need in carrying vast numbers of reserve players.

As the game wore on, several A&M players were injured (in an era when padding was frowned upon), and the team was running out of substitutes by halftime. Coach Dana X. Bible remembered that a former player, E. King Gill, was watching from the stands and press box, and called him to the bench. He was asked to suit up for the second half and did so, standing ready in case he was needed. He did not play in the game, but his simple willingness to be there in time of need became an enduring tradition.

During a game at Kyle Field, A&M students stand -- they do NOT sit down -- in a nod to Gill's standing ready to help the team. Furthermore, a squad of students to serve as walk-on (non-scholarship) players is recruited before every season. One of these players plays on every Aggie kickoff, wearing jersey number 12, and the stadium celebrates whenever the 12th Man makes the tackle of the opposing returner.