A cycloidal gear pair reducer including an input shaft, mounted in a reducer casing such that it rotates about an axis referred to as “main axis”, an eccentric member carried by the input shaft and rotated by the latter, at least one cycloidal disc rotatably mounted on the eccentric member and having a cycloidal toothing, at least one ring gear provided with a receiving toothing with which the cycloidal toothing of the cycloidal disc meshes, and an output shaft, separate from the input shaft, arranged to be rotated by the cycloidal disc; the cycloidal toothing of the disc and the corresponding receiving toothing of the ring gear being helical.