In American football, a spy is a defensive player assigned to cover an offensive backfield player man-to-man when they are expected to engage in a running play, but the offensive player does not run with the ball immediately.

This strategy is generally used with "dual-threat" quarterbacks who may be expected to run on their own after expected passing plays break down, with the defensive player floating near the line of scrimmage, following the quarterback's movements.^[1]

Spies are also sometimes known as "keys".^[2] Generally, spies are linebackers.^[3]