Linebacker (LB) is a playing position in gridiron football. Linebackers are members of the defensive team, and line up three to five yards behind the line of scrimmage and the defensive linemen. They are the "middle ground" of defenders, playing closer to the line of scrimmage than the defensive backs (secondary), but farther back than the defensive linemen.

As such, linebackers play a hybrid role and are often the most versatile players on the defensive side of the ball; they can be asked to play roles similar to either a defensive lineman (such as stopping the runner on a running play) or a defensive back (such as dropping back into pass coverage). How linebackers play their positions depends on the defensive alignment, the philosophy of the coaching staff, and the particular play the offense may call.

Linebackers are divided into middle linebackers, sometimes called inside linebackers, and outside linebackers. The middle linebacker, often called "Mike", is frequently the "quarterback of the defense". His central role on the field means he is in the best position to call defensive plays and direct shifts and changes based on what the offense is doing. Outside linebackers are often in a position to blitz, a defensive maneuver where the player rushes into the offensive backfield to disrupt a running play or sack the quarterback on passing plays. Besides "Mike", other nicknames such as "Sam", "Will", or "Jack", are commonly used to refer to various linebacking roles.

Historically, some of the most impactful defensive players, such as Pro Football Hall of Fame members Lawrence Taylor, Dick Butkus, Ray Nitschke, Chuck Bednarik, Mike Singletary, and Ray Lewis, were linebackers.