Trim tabs are small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft, used to control the trim of the controls, i.e. to counteract hydro- or aerodynamic forces and stabilise the boat or aircraft in a particular desired attitude without the need for the operator to constantly apply a control force. This is done by adjusting the angle of the tab relative to the larger surface.

Changing the setting of a trim tab adjusts the neutral or resting position of a control surface (such as an elevator or rudder). As the desired position of a control surface changes (corresponding mainly to different speeds), an adjustable trim tab will allow the operator to reduce the manual force required to maintain that position—to zero, if desired. Thus the trim tab acts as a servo tab. Because the center of pressure of the trim tab is farther away from the axis of rotation of the control surface than the center of pressure of the control surface, the moment generated by the tab can match the moment generated by the control surface. The position of the control surface on its axis will change until the torques from the control surface and the trim surface balance each other.

Planing boats or boats that operate at speeds close to planing will often have trim tabs on their engine lower unit or attached to the transom. Adjusting them up or down alters the pitch attitude of the boat while under way, variously balancing speed, weight distribution, and sea conditions.

Trim tabs are variously integrated into the rudder, elevators, and ailerons of a fixed-wing aircraft. As such, they are elements of an aircraft's system for allowing the pilot to control and maintain airspeed with a minimum of inputs and mental concentration. Many newer aircraft, especially jet aircraft, have electric trim controls.

Elevator trim frees the pilot from exerting constant force on the pitch controls, by adjusting trim control (often in the form of a vertical wheel) to cancel out control forces for a given airspeed and weight distribution. Typically, when this wheel is rotated up (or lever raised) the aircraft's nose pitches down; rotating it down (or depressing the lever) lowers the tail and raises the nose.

Many airplanes also have rudder and/or aileron trim systems. On some, the rudder trim tab is hinged and adjustable during flight; on others it is only adjustable on the ground (to lessen the need for the pilot to push the rudder pedal constantly to overcome the left-turning tendencies of many prop-driven aircraft).

Most fixed-wing aircraft have a trim tab on the elevator. However, alternative means of controlling the speed and attitude of the aircraft are sometimes used, including:

When a servo tab is employed, it is moved into the slipstream opposite to the control surface's desired deflection. For example, in order to trim an elevator to hold the nose down, the elevator's trim tab will actually rise up into the slipstream. The increased pressure on top of the trim tab surface caused by raising it will then deflect the entire elevator slab down slightly, causing the tail to rise and the aircraft's nose to move down. In the case of an aircraft where deployment of high-lift devices (flaps) would significantly alter the longitudinal trim, a supplementary trim tab is arranged to simultaneously deploy with the flaps so that pitch attitude is not markedly changed.