A lens module includes a base, a drive unit, a speed reduction unit, an outer barrel, and a lens unit. The speed reduction unit is coupled to and driven rotatably by the drive unit. The outer barrel is mounted to the base and has a barrel wall that surrounds an axis, that defines a receiving space, and that has an inner wall surface formed with an internally threaded portion. The lens unit includes a lens barrel that defines a chamber, and at least one lens disposed in the lens barrel. The lens barrel has an externally threaded portion that extends into the outer barrel to engage threadedly with the internally threaded portion of the barrel wall, and a driven portion driven rotatably by the speed reduction unit.