A cutter bar lockout mechanism for a header of an agricultural vehicle. The cutter bar lockout mechanism includes a shaft having a first end that is coupled to a frame of the header, and a second end that is positioned either on, within or adjacent a cutter bar support arm (arm) of the header. An upper stop is either mounted to the shaft or forms part of the shaft and is configured to selectively interact with an upper surface of the arm. A lower stop is either mounted to the shaft or forms part of the shaft and is configured to selectively interact with a lower surface of the arm. The shaft is movable relative to both the frame and the arm between an unlocked position and a locked position.