The tail rotor, or anti-torque rotor, is a smaller rotor mounted so that it rotates vertically or near-vertically at the end of the tail of a traditional single-rotor helicopter. The tail rotor's position and distance from the center of gravity allow it to develop thrust in the same direction as the main rotor's rotation, to counter the torque effect created by the main rotor. Tail rotors are simpler than main rotors since they require only collective changes in pitch to vary thrust. The pitch of the tail rotor blades is adjustable by the pilot via the anti-torque pedals, which also provide directional control by allowing the pilot to rotate the helicopter around its vertical axis.

The tail rotor drive system consists of a shaft powered from the main transmission and a gearbox mounted at the end of the tail boom. The drive shaft may consist of one long shaft or a series of shorter shafts connected at both ends with flexible couplings, that allow the drive shaft to flex with the tail boom. The gearbox at the end of the tailboom provides an angled drive for the tail rotor and may also include gearing to adjust the output to the optimum rotational speed for the tail rotor, measured in rotations per minute (RPM). On larger helicopters with a tail pylon, intermediate gearboxes are used to transition the tail rotor drive shaft from along the tailboom to the top of the pylon. The tail rotor pylon may also serve as a vertical stabilizing airfoil, to alleviate the power requirement for the tail rotor in forward flight. The tail rotor pylon may also serve to provide limited antitorque within certain airspeed ranges, in the event that the tail rotor or the tail rotor flight controls fail.