Third wheel is one of the three wheels present in the gear train of a mechanical watch. It is located in the middle of the gear train between the centre wheel and the fourth wheel.

The third wheel can be identified from the center wheel and the fourth wheel by its intermediate dimension but also by its pivots generally of the same dimension on the mainplate side and the bridge side.

In a traditional movement it can sometimes be located quite easily because it is one of the few wheels that on the movement side turns clockwise.

Although often called third wheel it represents in reality a mobile composed of two parts, a wheel generally in brass and a pinion and its pivots in steel.